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Item 1. Business

2023-03-02 compared with 2022-03-01 · 498 added, 569 removed, 696 unchanged (61% of the section changed)

We are a clinical-stage biotechnology company focused on harnessing the immune system to achieve deep and durable clinical responses to improve the lives of patients with cancer.

We have built our geographically precise solutions, or GPS, platform to rapidly engineer novel molecules, including cytokines and other biologics, that are designed to optimize their therapeutic index by geographically localizing their activity inside tumors.

Current immuno-oncology, or I-O, therapies have curative potential for patients with cancer;

however, their potential is significantly curtailed by systemic toxicity that results from activity of the therapeutic molecule outside the tumor microenvironment, or TME.

Our molecules are engineered to localize activity within the TME with minimal systemic effects, resulting in the potential to achieve enhanced anti-tumor activity.

We are advancing a number of geographically precise, or tumor-selective, agents through preclinical and clinical development.

Our most advanced product candidates are currently in clinical development:

XTX101, an anti-cytotoxic T-lymphocyte-associated protein 4, or anti-CTLA-4, monoclonal antibody, or mAb, and XTX202, an interleukin 2, or IL-2, therapy.

We are also advancing our tumor-selective IL-12 product candidate, XTX301, with the goal of submitting an investigational new drug application, or IND, to the U.S.

Food and Drug Administration, or FDA, in the second half of 2022 for evaluation in patients with solid tumors, and we are currently pursuing preclinical studies for our tumor-selective IL-15 product candidate, XTX401.

We also plan to continue to leverage our GPS platform to develop additional immunotherapies, including product candidates with a range of tumor targeting approaches.

XTX101 is a clinical-stage, tumor-selective anti-CTLA-4 mAb that is designed to improve upon the therapeutic index of existing anti-CTLA-4 therapies by localizing its activity in the TME.

Leveraging our GPS platform, we have engineered XTX101 to enhance the desirable features of an anti-CTLA-4 antibody while addressing its known liabilities.

In September 2021, we initiated a Phase 1/2 trial to evaluate XTX101 in patients with advanced solid tumors, and enrollment is ongoing.

The Phase 1 portion of the trial is a first-in-human, multi-center, open-label trial designed to evaluate the safety and tolerability of multiple ascending doses of XTX101 with the goal of establishing a recommended Phase 2 dose, or RP2D, for XTX101 both as a monotherapy and in combination with KEYTRUDA ® (pembrolizumab), an anti-programmed cell death protein 1, or anti-PD-1.

We expect to present preliminary data for the monotherapy cohort for this trial in the middle of 2022 and preliminary data for the combination cohort for this trial in the second half of 2022.

In May 2021, we entered into a clinical trial collaboration and supply agreement with Merck & Co., Inc., known as MSD outside the United States and Canada, to explore the therapeutic potential of XTX101 in combination with pembrolizumab in patients with advanced solid tumors.

In preclinical studies, we observed that XTX101 was activated ex vivo using human tumor samples showing cleavage of the masking domain.

In mouse models, XTX101 demonstrated anti-tumor activity at 0.3 mg/kg, similar to an ipilimumab analogue at 3 mg/kg and showed tolerability at doses up to 3 mg/kg in non-human primates, or NHPs.

XTX202 is an engineered form of IL-2 that is masked with a protein domain to prevent binding activity until the protein domain is cleaved off by TME-associated proteases.

In January 2022, we initiated a Phase 1/2 trial to evaluate XTX202 in patients with solid tumors, and enrollment is ongoing.

The Phase 1 portion of the trial is a first-in-human, multi-center, open-label trial designed to evaluate the safety and tolerability of multiple ascending doses of XTX202 with the goal of establishing a RP2D for XTX202 as a monotherapy in patients with advanced solid tumors.

We expect to present preliminary Phase 1 data in the second half of 2022.

We aim to establish safety proof-of-concept, or POC, by showing XTX202 has the ability to achieve target exposure at or below the RP2D without leading to significant adverse events, or AEs, in patients.

In addition, assuming we successfully complete the Phase 1 trial and determine the RP2D for XTX202, we plan to initiate Phase 2 expansion cohorts with XTX202 monotherapy to determine the objective response rate in RCC and melanoma patients who previously received an anti-PD-1 treatment regimen.

Subject to the response rate observed in the POC phase and discussions with regulatory authorities, we plan to initiate non-randomized monotherapy trials with the potential for accelerated approval.

In addition, we plan to initiate one or more additional Phase 1 trials aimed at demonstrating that we can combine XTX202 at its RP2D with a therapeutic dose of current standard-of-care treatments.

In preclinical studies, we observed that XTX202 was activated in a protease-dependent manner, including in preclinical studies using human tumor samples ex vivo showing XTX202 was able to cleave the masking domain, exhibited tumor growth inhibition and was well-tolerated in NHP models up to 10 mg/kg.

Leveraging our experience, we are applying our GPS platform to known cytokines that we believe have meaningful therapeutic potential, but that have been unable to achieve regulatory approval.

We are developing XTX301, our engineered tumor-selective IL-12 molecule, with the goal of submitting an IND in the second half of 2022, and we are currently pursuing preclinical studies for XTX401, our tumor-selective IL-15 product candidate.

Both of these cytokine product candidates exhibited tumor-selective activity with minimal peripheral effects in preclinical models, showed tumor growth inhibition in mouse models and were well-tolerated in multi-dose studies in NHP models.

We are a clinical-stage biotechnology company discovering and developing tumor-activated immuno-oncology, or I-O, therapies with the goal of significantly improving outcomes for people living with cancer without the systemic side effects of current I-O treatments.

We are leveraging our proprietary geographically precise solutions, or GPS, platform to build a pipeline of novel, tumor-activated molecules, including cytokines and other biologics, which are designed to optimize their therapeutic index by localizing anti-tumor activity within the tumor microenvironment.

Current I-O therapies have curative potential for patients with cancer;

however, their potential is significantly curtailed by systemic toxicity that results from activity of the therapeutic molecule outside the tumor microenvironment.

Our molecules are engineered to localize activity within the tumor microenvironment with minimal systemic effects, resulting in the potential to achieve enhanced anti-tumor activity and increasing the population of patients who may be eligible to receive our medicines.

Our most advanced tumor-activated, clinical-stage product candidates are XTX202, an interleukin 2, or IL-2, therapy, XTX301, an interleukin 12, or IL-12, therapy and XTX101, an Fc-enhanced, anti-CTLA-4 monoclonal antibody, or mAb.

In addition to our clinical-stage product candidates, we are continuing to leverage our GPS platform and expertise in developing tumor-activated I-O therapies as we seek to expand our pipeline of discovery-stage programs and develop additional tumor-activated immunotherapies, including product candidates with a range of tumor targeting approaches.

XTX202 is an investigational tumor-activated beta-gamma biased (non-alpha), engineered IL-2 molecule designed to potently stimulate CD8+ effector T cells and natural killer, or NK, cells without concomitant stimulation of regulatory T cells when activated (unmasked) in the tumor microenvironment.

We are currently evaluating XTX202 in an ongoing Phase 1 clinical trial in patients with advanced solid tumors.

The Phase 1 trial is a first-in-human, multi-center, open-label trial designed to evaluate the safety and tolerability of multiple ascending doses of XTX202 with the goal of establishing a recommended Phase 2 dose, or RP2D, for XTX202 as a monotherapy in patients with advanced solid tumors.

We recently completed dosing patients at the 1 mg/kg dose level (dose level four) and are currently enrolling patients at the 1.4 mg/kg dose level (dose level five) in monotherapy dose-escalation (Part 1A) of the clinical trial.

Importantly, no signs of vascular leak syndrome, or VLS, including hypotension or decreases in albumin (an early sign of VLS) or hemodynamic compromise, have been observed in patients to date.

Monotherapy dose expansion (Part 1B) of the Phase 1 clinical trial is open for enrollment, and we plan to initiate patient enrollment in a Phase 2 clinical trial evaluating XTX202 as a monotherapy in patients with unresectable or metastatic melanoma and metastatic renal cell carcinoma, or RCC, in April 2023.

In addition, we plan to report preliminary anti-tumor activity, safety, pharmacokinetic, or PK, and pharmacodynamic, or PD, data from the Phase 1/2 clinical trial in the third quarter of 2023, which we anticipate will include approximately 15-20 patients across a range of solid tumors treated at the 1 mg/kg dose or higher across all cohorts in the Phase 1/2 clinical trial.

XTX301 is an investigational tumor-activated, engineered IL-12 molecule designed to potently stimulate anti-tumor immunity and reprogram the tumor microenvironment of poorly immunogenic “cold” tumors towards an inflamed, or “hot,” state.

In November 2022, we announced clearance of our investigational new drug application, or IND, by the U.S.

Food and Drug Administration, or FDA, for XTX301 for evaluation in patients with advanced solid tumors.

We recently opened clinical trial sites and are actively screening patients for enrollment at a starting dose of 5.0 μg/kg (0.005 mg/kg) in monotherapy dose-escalation for our Phase 1 clinical trial evaluating the safety and tolerability of XTX301 in patients with advanced solid tumors.

We anticipate reporting preliminary safety data from the Phase 1 clinical trial into at least the third dose level in the fourth quarter of 2023.

XTX101 is an investigational Fc-enhanced, tumor-activated anti-CTLA-4 mAb designed to deplete regulatory T cells when activated (unmasked) in the tumor microenvironment and improve upon the therapeutic index of existing anti-CTLA-4 therapies.

We are currently evaluating XTX101 in an ongoing Phase 1 clinical trial in patients with advanced solid tumors.

The Phase 1 clinical trial is a first in-human, multi-center, open-label trial designed to evaluate the safety and tolerability of XTX101 for the treatment of adult patients with advanced solid tumors.

We recently completed enrollment in monotherapy dose-escalation (Part 1A) of the Phase 1 trial, and enrollment in monotherapy dose-expansion (Part 1B) of the Phase 1 trial is ongoing.

In addition, we have determined an RP2D of 150 mg once every six weeks, or Q6W.

anticipate reporting preliminary safety, PK, PD and anti-tumor activity data from the Phase 1 clinical trial in the second quarter of 2023.

As previously announced, we plan to continue to explore strategic opportunities to advance XTX101 with a partner beyond the current Phase 1 clinical trial.

In addition to our most advanced product candidates, we believe our proprietary GPS platform has the potential to develop additional product candidates using a range of approaches to achieve tumor-activation and derive a clinically meaningful improvement in therapeutic index.

We currently have worldwide development and commercialization rights to all of our product candidates.

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Although cytokines have demonstrated compelling clinical efficacy in certain tumors, including the ability to generate sustained complete responses, or CRs, in a subset of patients, their use has been limited by severe systemic toxicity.

Similar to cytokines, checkpoint inhibitors have shown the potential to provide meaningful improvements in survival for patients with cancer, but the utilization of these therapies, beyond those that target the immune proteins PD-1 or PD(L)1, is also limited largely by toxicity.

Our goal is to overcome the limitations of current I-O therapies by developing products with an improved efficacy-to-toxicity ratio, or therapeutic index.

The toxicities for cytokines and checkpoint inhibitors stem from their activity outside of the TME.

Our GPS platform is designed to overcome these systemic toxicities by creating tumor-selective molecules and unleashing the activity of cytokines and checkpoint inhibitors in the TME.

These molecules are intended to be inactive until they reach the TME, where they are activated, resulting in localized clinical activity with minimal dose-limiting toxicities.

The toxicities for cytokines and checkpoint inhibitors stem from their activity outside of the tumor microenvironment.

Our GPS platform is designed to overcome these systemic toxicities by creating tumor-activated molecules and unleashing the activity of cytokines and checkpoint inhibitors in the tumor microenvironment.

These molecules are intended to be inactive until they reach the tumor microenvironment, where they are activated, resulting in localized clinical activity with minimal dose-limiting toxicities.

To achieve this tumor selectivity, we apply our GPS platform, which includes engineered features and a proprietary protein masking technology that render our molecules inactive until reaching the tumor.

Our GPS platform also enables regulated pharmacokinetics, or PK, and protease-dependent activation, resulting in geographically localized anti-tumor activity.

Our GPS platform also enables PK and protease-dependent activation, resulting in geographically localized anti-tumor activity.

The engineered features are designed to ensure that our product candidates are stable molecules with well-understood properties and a reproducible manufacturing approach.

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By leveraging our GPS platform, we aim to discover, develop and, ultimately, commercialize I-O therapies that overcome the known limitations of today’s approaches and provide effective, tolerable and durable therapeutic options for patients and their physicians.

In order to achieve our goal, the key elements of our strategy are to:

● Efficiently progress our most advanced cytokine product candidate, XTX202, through clinical development for multiple cancer indications as both a monotherapy and a combination agent.

Using our GPS platform, we engineered and are developing XTX202, a novel, modified form of IL-2 that is masked with a protein domain to prevent binding activity until cleaved off by TME-associated proteases.

Due to its mechanism of action and opportunity to deliver tumor-directed activity while minimizing the risk of off-tumor effects, we plan to explore the therapeutic utility of XTX202 for the treatment of solid tumor indications.

In January 2022, we initiated a Phase 1/2 trial to evaluate XTX202 in patients with solid tumors, and enrollment is ongoing.

We expect to present preliminary Phase 1 data in the second half of 2022.

If we receive positive findings from the Phase 1/2 trial, and subject to discussions with regulatory authorities, we intend to efficiently advance XTX202

into registration-enabling clinical trials both as a monotherapy and as a combination therapy initially for the treatment of renal cell carcinoma, or RCC, and melanoma prior to potential expansion into additional cancer indications.

● Execute our Phase 1/2 development program for XTX101 and, if successful, transition to randomized registration-enabling trials in a range of solid tumor indications.

In September 2021, we initiated our Phase 1/2 trial for XTX101 in patients with advanced solid tumors, and enrollment is ongoing.

This trial is designed to evaluate the safety of XTX101 as monotherapy and in combination with pembrolizumab.

We expect to present preliminary data for the monotherapy cohort for this trial in the middle of 2022 and preliminary data for the combination cohort for this trial in the second half of 2022.

Following establishment of a RP2D for XTX101 in combination with pembrolizumab, we plan to initiate registration-enabling trials of XTX101 in combination with an anti-PD(L)-1 in a range of advanced solid tumor types, including melanoma, RCC, non-small cell lung cancer, or NSCLC, and prostate cancer based on the established role of ipilimumab in these indications.

Given the broad range of potential indications, we plan to explore opportunities for strategic collaborations to maximize the full potential of XTX101 and support its future commercialization, if approved.

● Advance our preclinical cytokine product candidates, XTX301 and XTX401, into clinical development.

Leveraging our experience, we are also applying our GPS platform to known cytokines that we believe have meaningful therapeutic potential but that have been unable to achieve regulatory approval to date.

As part of our strategy, we are developing XTX301 and XTX401, which are engineered tumor-selective IL-12 and IL-15 molecules, respectively.

We are advancing XTX301 with the goal of submitting an IND in the second half of 2022 for evaluation in patients with solid tumors, and we are currently pursuing preclinical studies for XTX401, our tumor-selective IL-15 product candidate.

In preclinical models, both product candidates exhibited tumor-selective activity with minimal peripheral effects.

In order to achieve these goals, the key elements of our strategy are to:

● Efficiently progress XTX202, our tumor-activated IL-2, through initial Phase 1 clinical proof-of-concept data, and if successful, expand clinical development for multiple cancer indications as a monotherapy and in combination with other therapies, including in combination with XTX301, our tumor-activated IL-12.

XTX202 is a tumor-activated beta-gamma biased (non-alpha), engineered IL-2 molecule designed to potently stimulate CD8+ effector T cells and NK cells without concomitant stimulation of regulatory T cells when activated (unmasked) in the tumor microenvironment.

We are currently evaluating XTX202 as a monotherapy in an ongoing Phase 1 clinical trial in patients with advanced solid tumors, and we anticipate initiating patient

enrollment in a Phase 2 clinical trial evaluating XTX202 as a monotherapy in patients with unresectable or metastatic melanoma and metastatic RCC in April 2023.

In addition, we plan to report preliminary anti-tumor activity, safety, PK and PD data from the Phase 1/2 clinical trial in the third quarter of 2023, which we anticipate will include approximately 15-20 patients across a range of solid tumors treated at the 1 mg/kg dose or higher across all cohorts in the Phase 1/2 clinical trial.

Subject to these interim trial results and establishing initial clinical proof-of-concept for XTX202, we plan to complete the Phase 2 trial evaluating XTX202 as monotherapy in patients with RCC and melanoma with the goal of determining the potential to conduct monotherapy registration-enabling trials for XTX202.

In parallel to the completion of the planned Phase 2 monotherapy trial, we plan to initiate a Phase 1/2 trial evaluating XTX202 in combination with an anti-PD-1/PD-L1 for the treatment of patients with non-small cell lung cancer, or NSCLC.

Subject to the results from this combination study, we anticipate initiating registration-enabling clinical trials for XTX202 in combination with an anti-PD-1/PD-L1 in patients with NSCLC.

We also plan to explore the potential for XTX202 in combination with other therapies, including combinations with XTX301, our tumor-activated IL-12, and potential collaborations with cell therapies or anti-cancer vaccines.

● Efficiently progress XTX301, our tumor-activated IL-12, through initial Phase 1 monotherapy dose-escalation, and if successful, expand clinical development for multiple “hot” and “cold” tumors as a monotherapy and in combination with other therapies.

IL-12 plays a key role in bridging innate and adaptive cellular immunity, making it a compelling target for immunotherapy.

However, life-threatening toxicity observed with systemically active IL-12, including severe liver toxicity, have limited the therapeutic potential of IL-12 agents.

XTX301, our tumor-activated IL-12, is designed to potently stimulate anti-tumor immunity and reprogram the tumor microenvironment of poorly immunogenic “cold” tumors towards an inflamed, or “hot,” state.

We anticipate reporting preliminary safety data from the Phase 1 clinical trial into at least the third dose level in the fourth quarter of 2023.

Subject to achieving a minimal target dose, we would continue monotherapy dose-escalation of the Phase 1 clinical trial and initiate treatment in cohorts of patients with both “cold” and “hot” tumors to determine the potential for anti-tumor activity.

Following an assessment of anti-tumor effect as a monotherapy in selected tumor types, we also plan to explore the combination potential of XTX301 with an anti-PD-1/PD-L1 and with XTX202, our tumor-activated IL-2 product candidate, prior to initiation of registration-enabling clinical trials.

Subject to the successful completion of trials showing the ability to combine XTX301 with other I-O agents and immune activation in “cold” tumors, we would plan to explore potential registration-enabling trials in both “hot” tumors, such as head and neck cancer and NSCLC, and in “cold” tumors, such as microsatellite stable colorectal cancer, or MSS CRC, and pancreatic cancer.

Report Phase 1 monotherapy data for XTX101 and explore strategic opportunities to advance XTX101 with a partner.

We are currently evaluating XTX101 in an ongoing Phase 1 clinical trial in patients with advanced solid tumors.

We recently completed enrollment in monotherapy dose-escalation (Part 1A) of the Phase 1 trial, and enrollment in monotherapy dose-expansion (Part 1B) of the Phase 1 trial is ongoing.

In addition, we have determined an RP2D of 150 mg Q6W, and we anticipate reporting preliminary safety, PK, PD and anti-tumor activity data from the Phase 1 clinical trial in the second quarter of 2023.

As previously announced, we plan to continue to explore strategic opportunities to advance XTX101 with a partner beyond the current Phase 1 clinical trial.

Broadly expand our portfolio by applying the versatility and reproducibility of our GPS platform to develop novel I-O therapies for cancer.

We have prioritized efforts to develop novel I-O therapies based on the therapeutic activity of I-O agents established in other clinical trials, while recognizing that the benefit of many of these I-O agents, particularly cytokines, has been historically hampered by issues of short half-life, poor bioavailability and significant toxicity.

By leveraging the insights and capabilities of our platform and our leadership team, we aim to systematically create novel molecules, including cytokines and other biologics, that overcome these challenges to safely localize their potent activity to the TME.

As a result, we intend to develop a number of additional product candidates using a range of tumor targeting approaches, with the goal of achieving a clinically meaningful improvement in their therapeutic index.

We plan to continue to invest in our innovative GPS platform and in our team to further expand our capabilities to engineer, manufacture and develop potentially more effective and less toxic therapies.

By leveraging the insights and capabilities of our GPS platform and our leadership team, we aim to systematically create novel molecules, including cytokines, mAbs and multi-functional biologics, that overcome these challenges to safely localize their potent activity to the tumor microenvironment.

We believe that our GPS platform, which enables us to develop molecules with the potential to trigger anti-tumor immunity while limiting systemic exposure to improve tolerability, is broadly applicable in oncology and has the potential to develop a number of additional product candidates using a range of tumor targeting approaches, with the goal of achieving a clinically meaningful improvement in their therapeutic index.

Build a fully-integrated I-O company by independently commercializing approved products in indications and key geographies where we believe we can maximize our product candidates’ value.

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We believe the collective components of our GPS platform and the reproducibility it enables in our drug discovery and development efforts present a meaningful opportunity for us to leverage our GPS platform not only in oncology but also in multiple therapeutic areas beyond oncology, such as in autoimmune and inflammatory diseases.

We plan to explore strategic collaborations that would enable us to accelerate the

development of additional product candidates or programs as well as expand our capabilities, pipeline opportunities and product offerings, particularly where a collaborator may have expertise or capabilities that are synergistic or additive to our own.

We plan to explore strategic collaborations that would enable us to accelerate the development of additional product candidates or programs as well as expand our capabilities, pipeline opportunities and product offerings, particularly where a collaborator may have expertise or capabilities that are synergistic or additive to our own.

The discovery of a role for immunotherapy in the treatment of cancer was made more than 100 years ago, when William Coley treated patients with heat-treated bacterial toxins, resulting in a profound anti-tumor effect in some of those patients.

Two of the most important mechanisms within I-O are checkpoint inhibitors and cytokines, with cytokine therapies having been introduced in the 1980s and checkpoint inhibitors in the period after 2011, when the first such product was approved.

Two of the most important mechanisms within I-O are cytokines and checkpoint inhibitors, with cytokine therapies having been introduced in the 1980s and checkpoint inhibitors in the period after 2011, when the first such product was approved.

Both therapeutic approaches are known to provide efficacy in terms of clinical responses and tumor shrinkage.

However, toxicities have limited the application of these therapies, resulting in the need to dose-reduce, dose-interrupt or discontinue many patients from treatment.

Immune checkpoint inhibitors are associated with immune-related AEs that may affect any organ system and may be life-threatening or fatal to patients.

Immune checkpoint inhibitors are associated with immune-related adverse events, or AEs, that may affect any organ system and may be life-threatening or fatal to patients.

Cytokines in particular are associated with broad ranging multi-organ toxicities that can be lethal and have limited the development of this class of potential therapies.

Anti-PD-1 checkpoint inhibitors have been used broadly because they generally achieve efficacy with minimal systemic toxicity, enabling their administration at their maximally effective doses.

Anti-PD-1 treatments have become the most widely utilized immunotherapy agent in oncology, with FDA approvals in more than a dozen separate tumor types and $27 billion in 2020 worldwide sales.

It is our mission to overcome the limitations of other checkpoint inhibitors, such as anti-CTLA-4, and cytokine therapies and make immunotherapies beyond anti-PD-1 treatments more accessible, efficacious and safe for patients with cancer.

Anti-PD-1/PD-L1 checkpoint inhibitors have been used broadly because they generally achieve efficacy with minimal systemic toxicity, enabling their administration at their maximally effective doses.

Anti-PD-1/PD-L1 treatments have become the most widely utilized immunotherapy agent in oncology, with FDA approvals in more than a dozen separate tumor types and $27 billion in worldwide sales in 2020.

Our mission is to overcome the limitations of cytokine therapies and checkpoint inhibitors, such as anti-CTLA-4, and make immunotherapies beyond anti-PD-1/PD-L1 treatments more accessible, efficacious and safe for patients with cancer.

The promise and limitations of cytokines

Cytokines are small proteins that carry messages between cells and serve as master regulators of the body’s response to inflammation and immune attack.

There are multiple cytokines, including interferon alpha (α), beta (β) and gamma (γ), as well as IL-2, which are approved in a range of oncology and non-oncology indications.

Interferon α was first approved in 1986 for the treatment of patients with hairy cell leukemia and was subsequently approved in a range of oncology indications including malignant melanoma, chronic myeloid leukemia and follicular lymphoma.

Aldesleukin, a high-dose IL-2 therapy, was first approved in 1992 as a monotherapy for patients with melanoma and RCC.

In addition, interferons have shown the potential for cytokines beyond oncology with approvals for the treatment of patients with multiple sclerosis, resulting in cytokines becoming a key treatment option for a range of conditions.

Cytokines are small signaling proteins that serve as master regulators of the body’s response to inflammation and immune attack.

There are multiple cytokines, including IL-2, which are approved in a range of oncology and non-oncology indications.

Aldesleukin, a high-dose IL-2 therapy, was first approved in 1992 as a monotherapy for patients with

melanoma and RCC.

In addition, cytokines have shown potential beyond oncology with approvals for the treatment of patients with multiple sclerosis, resulting in cytokines becoming a key treatment option for a range of conditions.

However, cytokines have not achieved therapeutic success in a broad population of patients because their use has been limited by severe toxicity, including fatal outcomes.

We believe XTX202, our tumor-selective IL-2 product candidate, has the potential to overcome these limitations and deliver the full clinical benefit of IL-2 without the dose-limiting toxicities associated with existing IL-2 treatments.

We believe XTX202, our tumor-activated IL-2 product candidate, and XTX301, our tumor-activated IL-12 product candidate, each have the potential to overcome these limitations and deliver a therapeutic dose with low systemic toxicity and the ability to achieve a broad therapeutic index.

The promise and limitations of checkpoint inhibitors

Checkpoint inhibitors have become mainstays in cancer therapy since the FDA approved ipilimumab, an anti-CTLA-4 therapy, in 2011.

As has been observed with cytokines, checkpoint inhibitors have shown the potential to provide meaningful improvements in survival for patients with cancer, but the utilization of these therapies has been limited largely by toxicities.

These toxicities, which can be life-threatening or fatal, have resulted in the need to dose-reduce, dose-interrupt or discontinue many patients from treatment.

As noted earlier, anti-PD-1 checkpoint inhibitors have been used broadly because they achieve efficacy with minimal systemic toxicity, enabling their administration at their maximally effective doses.

The clinical benefit of CTLA-4 blockade to patients with cancer is well-established;

however, efficacy of current therapies is impaired by dose-limiting toxicities arising from systemic immune activation.

To date, anti-PD-1/PD-L1 checkpoint inhibitors have been used broadly due to their ability to achieve efficacy with minimal systemic toxicity, enabling their administration at their maximally effective doses.

In contrast, while the clinical benefit of CTLA-4 blockade to patients with cancer is well-established, the efficacy of current CTLA-4 therapies is impaired by dose-limiting toxicities arising from systemic immune activation.

This has reduced the use of anti-CTLA-4 mAbs both as a monotherapy and in combination therapy.

We believe XTX101, our tumor-selective anti-CTLA-4 mAb, has the potential to overcome these limitations and deliver the full clinical benefit of anti-CTLA-4 mechanisms without the dose-limiting toxicities associated with existing CTLA-4 treatments.

We believe XTX101, our Fc-enhanced, tumor-activated anti-CTLA-4, has the potential to overcome these limitations and deliver the full clinical benefit of anti-CTLA-4 mechanisms without the dose-limiting toxicities associated with existing CTLA-4 treatments.

I-O Combinations

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The severe toxicity of IL-2 has limited the ability to combine IL-2 with other cancer treatments without compromising the dose administered.

Ipilimumab has been combined with anti-PD-1 agents.

For example, data from third-party clinical trials has demonstrated that the combination of ipilimumab with nivolumab, which targets the immune checkpoint protein PD-1, was associated with improved clinical outcomes, but it was limited by significantly higher risk of all-grade and high-grade immune-related AEs such as pruritus, rash, diarrhea, colitis, elevation of the liver enzyme alanine transaminase, known as ALT, hyperthyroidism, hypophysitis and pneumonitis.

Of note, combination therapy generally requires use of low dose ipilimumab at 1 mg/kg rather than the more efficacious dose of 10mg/kg.

Similarly, data from third-party clinical trials has demonstrated that the combination of ipilimumab, an anti-CTLA-4 therapy, with nivolumab, which targets the immune checkpoint protein PD-1, was associated with improved clinical outcomes, but it was limited by significantly higher risk of all-grade and high-grade immune-related AEs such as pruritus, rash, diarrhea, colitis, elevation of the liver enzyme alanine transaminase, known as ALT, hyperthyroidism, hypophysitis and pneumonitis.

Importantly, combination therapy generally requires use of low dose ipilimumab at 1 mg/kg rather than the more efficacious dose of 10 mg/kg.

Even at the lower dose, ipilimumab combination therapy is poorly tolerated, with AEs causing up to 80% of patients to discontinue treatment, up to 50% of patients requiring emergency room visits and up to 36% of patients requiring hospitalization.

The potential of our GPS-enabled molecules to minimize the systemic toxicity of I-O could allow us to combine I-O agents to meaningfully improve survival in a broader range of tumor types.

The potential of our tumor-activated molecules to minimize the systemic toxicity of I-O could allow us to combine I-O agents to meaningfully improve survival in a broader range of tumor types.

Our Solution:

Our GPS Platform Enables Tumor-Selective I-O Molecules Designed to Optimize Their Therapeutic Index

Our GPS Platform Enables Tumor-Activated I-O Molecules Designed to Optimize Their Therapeutic Index

I-O therapies have curative potential for patients with cancer.

However, to date this potential has been significantly curtailed by dose-limiting toxicities that result from activity of the therapeutic molecule outside the TME.

We believe that geographic localization of the activity of I-O agents to the TME can overcome these dose-limiting toxicities and enable maximal therapeutic benefit for patients.

Tumor-selective activity could be achieved by harnessing unique characteristics of the TME to activate therapeutic molecules locally that have minimal or non-detectable levels of activity outside of the TME.

Matrix metalloproteases, or MMPs, are enzymes involved in protein degradation that are essential for tumor growth and metastasis because they regulate key processes within the TME, including growth, survival, angiogenesis, invasion and metastasis.

MMPs are preferentially active in the TME by comparison to non-tumor organs or tissues.

As a result, we believe MMP activity can be leveraged to activate molecules within the TME that have minimal or non-detectable levels of activity outside the TME.

This has been validated by the MMP activated imaging agent AVB-620, which selectively highlights tumor tissue in human clinical trials and has received Breakthrough Therapy Designation from the FDA as a diagnostic.

Similarly, our GPS platform is designed to harness MMPs to activate therapeutic molecules selectively within the TME while allowing them to have minimal or non-detectable levels of activity outside of the TME.

Our GPS platform enables us to engineer a broad range of immune-modulatory molecules, including cytokines and antibodies, that contain masking domains that minimize their activity outside of the TME.

The molecules are then designed to be turned on selectively in the TME where they are activated by the unique conditions in the TME, including the preferential activity of MMPs.

Specifically, MMPs cleave a linker that connects the masking protein domain to the active agent.

This separates the mask from the active agent, enabling the unmasked agent to promote an anti-tumor response within the TME.

Key features of our tumor-selective molecules exemplify the engineering approach that underpins our GPS platform.

Each feature contributes to multiple characteristics of the molecule that are designed to enable tumor selective biological activity and tumor growth inhibition while minimizing toxicity outside of the TME.

The general architecture of a GPS-enabled molecule is:

However, this potential has been significantly curtailed to date by dose-limiting toxicities that result from activity of the therapeutic molecule outside the tumor microenvironment.

We believe that geographic localization of the activity of I-O agents to the tumor microenvironment can overcome these dose-limiting toxicities and enable maximal therapeutic benefit for patients.

Tumor-selective activation could be achieved by harnessing unique characteristics of the tumor microenvironment to activate therapeutic molecules locally that have minimal or non-detectable levels of activity outside of the tumor microenvironment.

Matrix metalloproteases, or MMPs, are enzymes involved in protein degradation that are essential for tumor growth and metastasis because they regulate key processes within the tumor microenvironment, including tumor growth, angiogenesis, invasion and metastasis.

MMPs are dysregulated in the tumor microenvironment, resulting in preferential activity of MMPs in the tumor microenvironment by comparison to non-tumor, healthy tissues.

As a result, we believe that our GPS platform,

which is designed to harness tumor MMP activity, can activate our molecules selectively within the tumor microenvironment while maintaining minimal or non-detectable levels of activity outside of the tumor microenvironment.

Our GPS platform enables us to engineer a broad range of immune-modulatory molecules, including cytokines, antibodies and multi-functional molecules, that contain masking domains designed to minimize their activity outside of the tumor microenvironment and turn on selectively in the tumor microenvironment where they are preferentially activated by tumor MMPs.

Specifically, MMPs enzymatically cleave a protease cleavage site incorporated in a peptide linker that connects the masking protein domain to the active agent.

This separates the mask from the active agent, enabling the unmasked agent to promote an anti-tumor response within the tumor microenvironment.

Key features of our tumor-activated molecules exemplify the engineering approach that underpins our GPS platform.

The general architecture of each of our tumor-activated molecules is:

a masking domain;

1 unchanged sentence

a protease cleavage site;

● solely for GPS-enabled cytokines, a half-life extension domain;

a half-life extension domain for cytokine molecules, as appropriate;

the active engineered molecule (i.e., an engineered cytokine or antibody).

The engineered features are designed to ensure that our product candidates are stable molecules with well-understood properties and a reproducible manufacturing approach.

In preclinical studies, we have successfully applied our GPS platform to cytokines and antibodies that regulate immune checkpoints in order to promote localized anti-tumor immune responses, while avoiding the dose-limiting toxicities associated with systemic immune responses.

In the graphic below, an illustrative cytokine product candidate on the left contains a masking domain that is released by protease cleavage.

When the linker sequences, which contain a protease-cleavage site, are cleaved by proteases, the masking domain is released, allowing the cytokine to bind to the target receptors.

Before cleavage by the MMP in the TME, the engineered cytokine has minimal or non-detectable levels of activity outside the TME.

Specifically, there is no binding to target receptors, and the molecule has a long half-life outside the TME.

After cleavage in the TME, the engineered cytokine is locally activated and has a short half-life.

Key Features of GPS-Enabled Cytokines

Similarly, in the graphic below, an illustrative antibody product candidate on the left contains a masking domain that is released by protease cleavage.

The masking domain is shown in tan and the active antibody is shown in orange.

When the linker sequences, shown in black, which contain a protease-cleavage site, shown in green, are cleaved by proteases, the masking domain is released, allowing the antibody to bind to the target receptors for cytokines.

Before cleavage by the MMP in the TME, the engineered antibody has minimal or non-detectable levels of activity outside the TME.

Specifically, there is no binding to target receptors.

After cleavage in the TME, the engineered antibody is locally activated.

Key Features of GPS-Enabled Antibodies

As shown in the graphics above, the structural components comprising our GPS platform work synergistically to improve the therapeutic index of our product candidates.

These design features also enhance intrinsic stability of the molecule and manufacturing.

● Masking Domain.

The masking domain is a protein domain that is selected for optimal masking outside of the TME and allows efficient activation upon cleavage by a protease within the TME.

Masking of the engineered molecule is designed to prevent binding to its cognate receptor on immune cells, thereby minimizing biological activity outside the TME.

In addition, the masking domain contributes to improved PK properties and a longer half-life for the inactive molecule by reducing target mediated drug disposition.

● Linker Sequences.

Linker sequences connect the engineered molecule and the mask to the half-life extension domain and are selected to optimize masking and stability of the molecule.

● Protease Cleavage Site.

A protease cleavage site within a linker sequence allows cleavage by a subset of proteases, particularly MMPs, unmasking the engineered molecule and allowing it to affect its engineered immune activity.

We tailor the protease site for optimal masking and subsequent activation of the engineered molecule in the TME.

Biochemical, cell-based and ex vivo functional studies have demonstrated, with a subset of MMPs, efficient and full cleavage and complete rescue of cytokine activity and selective activation of the masked engineered molecule in the TME by comparison to non-tumor tissues.

These data have been further substantiated by in vivo studies in mice and NHPs that demonstrate engineered molecule activity in the TME, but not outside of the TME, and also demonstrate the tumor-selective activation of the protease sites incorporated into GPS-enabled engineered molecule.

● Half-Life Extension Domain.

The half-life extension domain is designed to overcome the short circulating half-life of native cytokines such that the inactive masked cytokine circulates for a prolonged period while outside of the TME.

For antibodies, a half-life extension is not required given their longer half-life.

In preclinical studies, we have successfully applied our GPS platform to cytokines, antibodies that regulate immune checkpoints and multi-functional molecules in order to promote localized anti-tumor immune responses, while avoiding the dose-limiting toxicities associated with systemic immune responses.

The graphic below illustrates XTX202, our tumor-activated IL-2, which contains a masking domain that is released by protease cleavage.

When the linker sequence, which contain a protease cleavage site, is cleaved by proteases, the masking domain is released, allowing the cytokine to bind to the target receptor.

Before cleavage by the MMP in the tumor microenvironment, the engineered cytokine has minimal or non-detectable levels of activity outside the tumor microenvironment.

Specifically, there is no binding to target receptors, and the molecule has a long half-life outside the tumor microenvironment.

After cleavage in the tumor microenvironment, the engineered cytokine is locally activated and has a relatively short half-life.

Key Features of Xilio’s Tumor-Activated Cytokines

We believe that the characteristics of our GPS platform described above enable the following key advantages:

● masking that takes advantage of multiple intra-molecular interactions, minimizing the risk of activity outside of the TME and therefore the risk of toxicity;

● engineering the active molecule such that unmasking in the TME promotes a potent anti-tumor immune response;

masking that takes advantage of multiple intra-molecular interactions, minimizing the risk of activity outside of the tumor microenvironment and therefore minimizing the risk of toxicity;

engineering the active molecule such that unmasking in the tumor microenvironment promotes a potent, localized anti-tumor immune response;

early consideration and incorporation of manufacturing and development aspects into the design of molecules to facilitate production of high-quality drug product for clinical use;

● half-life extending inactive cytokines to support administration to patients on a schedule consistent with other biologics agents;

● locally activating cytokine molecules that have a short half-life in the TME, which minimizes the risk of the released cytokine exhibiting activity outside of the TME and, therefore, further reduces the risk of toxicity.

We have shown preclinical validation of the ability of our GPS platform to develop tumor-selective antibodies and cytokines, as evidenced by our tumor-selective anti-CTLA-4 antibody, XTX101, and our tumor-selective IL-2, XTX202.

In preclinical studies, each of these product candidates has exhibited tumor-selective biological activity, tumor growth inhibition and minimal to no toxicity outside of the TME.

In addition, the reproducibility of these data, as evidenced by tumor-selective activity observed in preclinical studies with our engineered IL-12 and IL-15 cytokines, XTX301 and XTX401, highlights the potential breadth of application of our GPS platform to multiple structurally diverse cytokines or antibodies.

We believe our GPS platform can be applied to numerous molecules, including those shown in the image below, which have potential as cancer therapies, but need localized activity within the TME to overcome the dose-limiting toxicities that result from activity outside of the TME.

Potential Applicability of Our GPS Platform

Leveraging our GPS platform, we are building a pipeline of tumor-selective cytokine and checkpoint inhibitor immunotherapies to treat cancer.

half-life extending inactive (masked) cytokines, as appropriate, to support administration to patients on a schedule consistent with other biologics agents;

locally activating cytokine molecules that have a short half-life in the tumor microenvironment, which minimizes the risk of the released cytokine exhibiting activity outside of the tumor microenvironment and, therefore, further reduces the risk of toxicity.

We have shown preclinical validation and promising early clinical evidence of the ability of our GPS platform to develop tumor-activated cytokines and antibodies.

In preclinical studies, each of our most advanced product candidates exhibited tumor-activated biological activity, tumor growth inhibition and minimal to no toxicity outside of the tumor microenvironment.

The reproducibility of these preclinical data highlights the potential breadth of application of our GPS platform to multiple structurally diverse cytokines, antibodies and multi-functional molecules.

In our Phase 1 trial for XTX202, our tumor-activated IL-2, we have demonstrated initial clinical validation of our GPS platform, as evidenced by preliminary intratumoral PD data for two patients treated with XTX202, each of whom had an optional on-treatment tumor biopsy and were the only two patients from whom a tumor biopsy analysis was available as March 1, 2023.

For each patient, the tumor sample featured increased numbers of stromal tumor infiltrating lymphocytes, or TILs, and increased frequency of CD8+ effector T cells among these TILs.

At the time of the tumor sample, these changes occurred in each patient in the absence of peripheral changes to CD8+ effector T cells, demonstrating preliminary evidence of tumor-selective activation.

We believe our GPS platform can be applied to numerous molecules, which have potential as cancer therapies, but require localized activity within the tumor microenvironment to overcome the dose-limiting toxicities that result from activity outside of the tumor microenvironment.

Leveraging our GPS platform, we are building a pipeline of tumor-activated cytokine, checkpoint inhibitor and multi-functional immunotherapies to treat cancer.

Our goal is to overcome the limitations of current I-O therapies by developing products with an improved therapeutic index.

Consistent with this goal, we selected molecules that have prior clinical validation demonstrating therapeutic benefit, but that have been limited by significant toxicities that we believe can be addressed with our approach.

We have worldwide development and commercialization rights to all of our product candidates.

The following chart summarizes our product candidates and anticipated upcoming milestones.

Plan to initially evaluate XTX101 as a monotherapy and as a combination therapy for the treatment of advanced solid tumors

Plan to initially evaluate XTX202 as a monotherapy and as a combination therapy for the treatment of RCC and melanoma prior to potential expansion into additional cancer indications

Our development strategy is focused on three important concepts:

(1) achieve an acceptable safety profile while maintaining dose levels sufficient to confer localized activity in the tumor;

(2) establish clinical POC as a monotherapy;

(3) demonstrate the combinability of our molecules so they can be explored and administered with relevant standard-of-care agents to potentially improve patient outcomes.

We currently have worldwide development and commercialization rights to all of our product candidates.

Disease Indication

Mechanism of Action

Stage of Development

Cytokine Programs

Phase 1 monotherapy dose-escalation (Part 1A) ongoing and monotherapy dose expansion (Part 1B) open for enrollment

Phase 1 clinical trial sites open and actively screening patients for enrollment

Undisclosed Research Program

Tumor-Activated

Multi-Functional

Discovery stage

Antibody Program

Enrollment in Phase 1 monotherapy dose-escalation (Part 1A) complete;

Phase 1 monotherapy dose-expansion (Part 1B) ongoing

Plan to initially evaluate XTX202 as a monotherapy in patients with melanoma and RCC prior to evaluating XTX202 in combination with an anti-PD-1/PD-L1 for the treatment of patients with NSCLC or potential expansion into additional cancer indications as a monotherapy or combination therapy.

Plan to initially evaluate XTX301 as a monotherapy in patients with advanced solid tumors.

Plan to explore opportunities for strategic collaborations to advance XTX101 with a partner beyond the current Phase 1 clinical trial.

Our development strategy for our clinical-stage, tumor-activated cytokine product candidates is focused on the following:

achieve an acceptable safety profile while maintaining dose levels sufficient to confer localized anti-tumor activity in the tumor microenvironment;

achieve clinical proof-of-mechanism for our GPS platform by showing tumor-selective activation, as demonstrated by increased tumor fighting immune cells in patient tumors, including CD8+ effector T cells and NK cells, without an increase in tumor fighting immune cells in the periphery;

establish clinical proof-of-concept for each product candidate, as demonstrated by monotherapy anti-tumor activity in patients treated at the target dose level for such product candidate;

demonstrate the ability to administer our product candidates in combination with other therapies, so they can be explored and administered with relevant standard-of-care agents to potentially improve patient outcomes.

Assuming we successfully achieve an improved therapeutic index during our initial clinical trials, we plan to proceed into an expansive set of registration-enabling clinical trials across a range of tumor types.

XTX101, our clinical-stage tumor-selective checkpoint inhibitor program

XTX101 is a clinical-stage, tumor-selective anti-CTLA-4 mAb that is designed to improve upon the therapeutic index of existing anti-CTLA-4 therapies.

Using our GPS platform, we have engineered XTX101 to enhance the desirable features of an anti-CTLA-4 antibody while minimizing its known limitations due to toxicity.

We approached this by using complimentary-determining region, or CDR, masking of the antibody, identifying an antibody with improved binding to CTLA-4 and engineering enhanced binding to the Fcγ receptors.

A CDR is part of the variable chains in antibodies and T cell receptors where molecules bind to their specific antigen.

In September 2021, we initiated our Phase 1/2 trial to evaluate

XTX101 in patients with advanced solid tumors, and enrollment is ongoing.

The Phase 1 portion of the trial is a first-in-human, multi-center, open-label trial designed to evaluate the safety and tolerability of multiple ascending doses of XTX101 with the goal of establishing an RP2D both as a monotherapy and in combination with KEYTRUDA ® (pembrolizumab), an anti-PD-1.

We expect to present preliminary data for the monotherapy cohort for this trial in the middle of 2022 and preliminary data for the combination cohort for this trial in the second half of 2022.

In May 2021, we entered into a clinical trial collaboration and supply agreement with Merck & Co., Inc., known as MSD outside the United States and Canada, to explore XTX101 in combination with pembrolizumab in patients with advanced solid tumors.

Given the significant size of the opportunity, including in melanoma, RCC, NSCLC and prostate cancer, and the trials required to fully develop XTX101, we plan to seek collaboration opportunities to develop XTX101.

We have evaluated XTX101 in several preclinical studies, and, while there can be no assurances that these results will be replicated in clinical trials, we have observed activation of XTX101 at our target doses in the TME leading to enhanced activity and an improved tolerability profile compared to an ipilimumab analog.

Background on CTLA-4

CTLA-4 is an immune checkpoint protein that is well-established as playing a central role in the development of tumors.

The scientific insight that led to the early development of CTLA-4 therapeutics is attributable to investigators recognizing CTLA-4 as a protein on T cells that acts as a brake on T cell activation.

By removing this brake, T cells were freed to attack cancer.

This work led to the development and FDA approval of ipilimumab, a CTLA-4 mAb, for the treatment of unresectable or metastatic melanoma at a dose of 3 mg/kg in 2011 and in additional indications in subsequent years.

While anti-CTLA-4 therapies such as ipilimumab have demonstrated meaningful efficacy across a range of tumor types, autoimmune toxicities have significantly limited their use to date.

Clinical trials have shown that 20% of ipilimumab-treated melanoma patients survive at least three years, and a subset survive for 10 years or longer.

Ipilimumab remains one of the most impactful drugs for these patients;

however, the number of patients who benefit from treatment with ipilimumab remains limited due to its toxicity.

Investigation of dose-response in two third-party clinical trials of melanoma patients showed that higher doses of ipilimumab are likely to increase the proportion of patients who benefit;

however, the increased dose also resulted in an unacceptable toxicity profile for most patients.

In a Phase 2 trial of ipilimumab conducted by Bristol-Myers Squibb Company, a dose range of 0.3 mg/kg to 10 mg/kg was tested and efficacy was measured both by response rate and by clinical outcome.

Both the response rate and median overall survival, or mOS, were higher at 10 mg/kg than at 3 mg/kg, with the 0.3 mg/kg dose determined as being ineffective.

The rate of severe AEs was 25% at the 10 mg/kg dose, 7% at the 3 mg/kg dose and 0% at the ineffective dose of 0.3 mg/kg.

Similarly, as shown in the table below, in a Phase 3 trial conducted by Bristol-Myers Squibb Company, mOS was higher at the 10 mg/kg dose but resulted in unacceptable toxicity for most patients.

Therefore, we believe that achieving a three-fold increase in therapeutic index would be transformational.

High-Dose Ipilimumab Improved Survival but Resulted in Unacceptable Toxicity

Adverse Events:

Gr 3/4 irAEs / disconts.

Ipilimumab has shown preliminary evidence of promising anti-tumor activity in a range of tumor types outside of its currently approved indications, but successful additional approvals have been limited due to its toxicity.

For example, ipilimumab has been observed to be more active when combined with the anti-PD-1 antibody nivolumab.

However, the combination of ipilimumab and nivolumab has been shown to cause a greatly increased rate of immune-related toxicity when compared to treatment with either ipilimumab or nivolumab as a monotherapy.

Clinical results from patients who express high-affinity FcγR polymorphisms have shown improved responses to ipilimumab, but efforts to improve the potency of the antibody have been limited by perceived toxicity risk.

There remains a critical need to develop safe and effective forms of anti-CTLA-4 mAbs that can achieve efficacious doses within the TME.

Our solution:

XTX101 is a tumor-selective anti-CTLA-4 mAb that is designed to improve upon the therapeutic index of existing anti-CTLA-4 therapies by overcoming potency and tolerability limitations.

Our goal is to demonstrate an improved safety profile enabling higher anti-CTLA-4 exposure in the tumor that will result in increased efficacy.

In preclinical studies, we have observed the following tolerability and activity profile of XTX-101:

● improved in vivo potency and the intra-tumoral pharmacodynamic, or PD, effects of XTX101 are consistent with the improved potency being a result of the higher affinity binding to the target CTLA-4 and enhanced IgG1-Fc effector function, which further improves checkpoint inhibition and enhances antibody-dependent cellular cytotoxicity to deplete immune-suppressive regulatory T cells, or Tregs, in the TME;

● reduced peripheral immune activity due to masking of the CDR sequences;

● activation by protease-dependent release of the masks, which acts selectively in the TME and minimizes toxicity associated with systemic immune activation.

XTX101 is designed to enhance the desirable features of an anti-CTLA-4 antibody while limiting known limitations of anti-CTLA-4 antibodies due to toxicity.

We expect this combination of features to result in an increased therapeutic index.

Overview of preclinical studies and data

We have examined XTX101 in several preclinical and IND-enabling studies that we believe have demonstrated the potential for XTX101 to have an enhanced activity and an improved tolerability profile compared to ipilimumab.

As summarized below, we believe that these studies collectively provide preclinical POC with an improved therapeutic index due to TME-dependent activation of XTX101 that, if replicated in clinical trials, could result in significant benefits to patients with a variety of different solid tumors.

● XTX101 showed tight control and full reliance on MMP activity for binding to CTLA-4 in an in vitro study using a sensitive surface plasmon residence, or SPR, assay.

● XTX101 was activated by human tumors in a study of 82 tumor biopsies across a variety of solid tumors that demonstrated 67% overall activation.

● XTX101 demonstrated activity similar to an ipilimumab analog in in vivo mouse models of bladder cancer tumor growth.

XTX101 dosed at 0.3 mg/kg and 1.0 mg/kg resulted in 2/8 CRs and 5/8 CRs, respectively.

● In the same mouse study, we observed that XTX101 induced an increase in CD8+ T cells within the tumor and a decrease in Tregs in the tumor, without increasing CD4+ T cells in the blood.

The ipilimumab analog had less activity than XTX101 in the tumor but did show an increase in CD4+ T cells in the blood.

● In a separate mouse study, the combination of XTX101 and an anti-PD-1 antibody showed robust tumor growth inhibition in excess of either drug as a monotherapy, with minimal systemic toxicity.

We observed the dependency of XTX101 on proteolytic cleavage to achieve binding using an SPR assay, which measures on-rate and off-rate of antibody binding to the target protein, allowing an accurate assessment of binding.

The data demonstrate tight control and full reliance on MMP activity for binding of XTX101 to CTLA-4.

The figure below shows protease-dependent activation of XTX101 in vitro using a biophysical assay.

The left panel shows the binding of non-masked anti-CTLA-4 mAb, XTX100, to CTLA-4 coated on the SPR chip.

Binding is indicated by a positive response measured in resonance units, or RUs.

The middle panel shows minimal binding of unmasked anti-CTLA-4 mAb, XTX101, under the same conditions.

The right panels show that treatment of masked XTX101 with MMPs restores binding to CTLA-4 in the SPR assay.

After Proteolytic Activation, Full Binding was Restored to XTX101

We further demonstrated the capacity of human tumors to activate XTX101 in a protease-dependent manner using fresh tumor biopsies obtained from human patients and assessing their ability to activate XTX101 and thus restore binding to CTLA-4.

XTX101 activation was observed for all tumor types evaluated.

The total number of tumors tested was 82, and overall, 67% demonstrated activation of XTX101.

The proportion of tumors for each cancer indication that demonstrated protease-dependent activation of XTX101 was:

Demonstrating

Activation of

Cancer Indication

In in vivo models of bladder cancer tumor growth, XTX101 showed activity superior to that of an ipilimumab analog.

MB49 cells were inoculated subcutaneously into C57BL/6-huCTLA-4 mice.

When tumors reached approximately 150 mm 3 , mice received a single intravenous, or IV, dose of each molecule at the doses indicated in the figure.

These were 0.3 mg/kg or 1.0 mg/kg for XTX101 and 0.3 mg/kg, 1.0 mg/kg and 3.0 mg/kg for the ipilimumab analog, which we produced to conduct these studies.

As shown in the figure below, we observed that XTX101 was more clinically active than the ipilimumab analog in the MB49 bladder cancer model.

The left panel shows the effect of different doses of XTX101 on tumor growth, with two CRs achieved with a dose of 0.3 mg/kg and five CRs achieved with a dose of 1.0 mg/kg.

The right panel shows the effect of different doses of the ipilimumab analog on tumor growth, with no CRs achieved with a dose of either 0.3 or 1.0 mg/kg.

XTX101 exhibited superior tumor growth inhibition compared to the ipilimumab analog.

A dose of 3 mg/kg of the ipilimumab analog was required to achieve similar activity and CR rate as XTX101 at 0.3 mg/kg, suggesting XTX101 has 10-fold higher potency than the ipilimumab analog.

Clinical Activity of XTX101 and Ipilimumab Analog in MB49 Tumor Mice

Further, as shown in the below figure, we observed that XTX101 induced an increase in CD8+ T cells within the tumor, as shown in the left panel, and a decrease in Tregs in the tumor, as shown in the middle panel.

We observed that XTX101 did not promote an increase in CD4+ T cells in the blood even at 3 mg/kg despite achieving complete responses at 0.3 mg/kg, suggesting tumor-selective activity of XTX101.

We further observed that the ipilimumab analog promoted an increase in CD4+ T cells in the blood at the 3 mg/kg dose required for efficient tumor growth inhibition in the blood, as shown in the right panel, demonstrating that the ipilimumab analog was active outside of the TME at doses required for activity.

XTX101 Demonstrated Tumor-Selective PD and Treg Depletion

The effect of the combination of XTX101 with anti-PD-1 antibody was evaluated in another preclinical study in which female C57BL/6-huCTLA4 mice, were implanted subcutaneously with MC38 cells.

When the tumors reached approximately 150 mm 3 on day zero, the mice were intravenously administered 10mg/kg isotype control antibody, 0.3 mg/kg of XTX101 or 10mg/kg anti-PD-1.

As shown in the figure and table below, XTX101 and the anti-PD-1 antibody each showed limited activity as a monotherapy.

However, the combination of XTX101 with the anti-PD-1 antibody showed robust tumor growth inhibition of 82%, including two out of eight animals achieving a CR.

Minimal toxicity was observed in animals treated with either monotherapy or the combination, suggesting that XTX101 can be effectively combined with an anti-PD-1 antibody without enhanced toxicity.

Single Dose Combination of XTX101 with anti-muPD-1 Enhanced Tumor

Growth Inhibition with No Impact on Body Weight

Isotype Control

XTX101 0.3 mg/kg

Anti-PD ‑ 1 10 mg/kg

0.3 mg/kg + anti-PD ‑ 1

Complete responses

In GLP toxicology studies completed in NHPs, XTX101 exhibited a highest non-severely toxic dose, or HNSTD, of 3 mg/kg in NHP supporting the targeted clinical dose for XTX101 in patients of 1-3 mg/kg.

In addition, the half-life for XTX101 was 92 to 208 hours in single dose studies in NHPs, supporting the potential for dosing once every three weeks.

Clinical Development Plan

In September 2021, we initiated a Phase 1/2 trial to evaluate XTX101 in patients with advanced solid tumors, and enrollment is ongoing.

The Phase 1 portion of the trial is a first-in-human, multi-center, open-label trial designed to evaluate the safety and tolerability of multiple ascending doses of XTX101 with the goal of establishing a RP2D for XTX101 both as a monotherapy and in combination with KEYTRUDA ® (pembrolizumab), an anti-PD-1.

In addition, we intend to evaluate evidence of anti-CTLA-4 PD activity in tumors from patients treated with XTX101.

To assess the anti-tumor PD effects, patients who have progressed on anti-PD-1/PD-L1 treatment but have not received prior ipilimumab will be treated with XTX101, then patients will have tumor biopsies to assess PD.

We expect to present preliminary data for the monotherapy cohort for this trial in the middle of 2022 and preliminary data for the combination cohort for this trial in the second half of 2022.

Based on the observed improved activity of XTX101 over current CTLA-4-directed therapeutics in preclinical studies, such as an ipilimumab analog, its higher affinity binding to the target CTLA-4 and enhanced Fc-driven effector function, we also plan to look for early signs of anti-tumor activity in this initial trial.

Finally, in the Phase 2 portion of the trial, we intend to evaluate XTX101 in combination with pembrolizumab in patients who have not previously been treated with an anti-PD-1/PD-L1.

The objective is to show that XTX101 dosage is not limited by the dose-limiting toxicities observed with ipilimumab when ipilimumab is given in combination with anti-PD-1 agents.

Our planned registration-enabling program for XTX101 will require randomized trials in combination with an anti-PD-1 or anti-PD-L1.

Given the significant size of the opportunity, including in melanoma, RCC, NSCLC and prostate

cancer, and the trials required to fully develop XTX101, we plan to seek potential strategic collaboration opportunities to develop XTX101.

The following illustrates the design of Phase 1 of our Phase 1/2 trial of XTX101:

XTX101 Phase 1 Trial to Establish POC with Monotherapy and Safety in Combination with anti-PD-1

Metastatic Melanoma Overview

Melanoma of the skin is the nineteenth most commonly occurring cancer in the United States with approximately 100,000 new cases and approximately 7,200 deaths projected in 2021.

The rates of melanoma have been rising rapidly and treatment is a major driver of healthcare costs.

Although the five-year survival rate from diagnosis for early melanoma is over 95%, this drops to approximately 25% for metastatic melanoma that has spread to distant sites.

Despite the FDA approval of 12 new melanoma therapies since 2011, treatment options for patients with stage III and IV melanoma remains suboptimal.

After surgical resection, adjuvant treatment with a targeted therapy or a checkpoint inhibitor may be initiated.

For patients with metastatic disease, treatments that target PD-1 and CTLA-4 have grown to dominate the treatment landscape.

In about 50% of patients with melanoma, the BRAF gene is mutated and a combination of a BRAF inhibitor and a MEK inhibitor—for example, dabrafenib plus trametinib—might be used;

however, typically patients will ultimately receive I-O treatment.

These new therapies have resulted in significant improvements of relapse-free survival in patients with melanoma, and in some cases, an improvement in overall survival is observed.

However, many patients still relapse and suffer disease progression, indicating there is an urgent need for additional therapeutic options.

Melanoma has the strongest rationale for combining an anti-PD-1 with an anti-CTLA-4, as clinical trials have demonstrated the combination of nivolumab (an anti-PD-1 agent) and ipilimumab (an anti-CTLA-4 agent) demonstrated statistically significant improvements in overall response rate, or ORR, and a strong trend for improved survival compared to nivolumab monotherapy.

Despite the strong efficacy data seen with the combination of an anti-PD-1 agent and an anti-CTLA-4 agent, monotherapy treatment with an anti-PD-1 remains a common standard of care in the treatment of melanoma patients due to the severe toxicity associated with combination treatment.

Assuming a successful Phase 1 dose escalation trial for XTX101, we plan to pursue a clinical trial of XTX101 in combination with pembrolizumab in melanoma patients who have previously not been treated with an anti-PD-1 to demonstrate clinical proof-of-concept in this patient population.

Potential Future Indications

We intend to explore XTX101 in additional solid tumor indications for which ipilimumab has demonstrated meaningful activity.

In addition to melanoma, ipilimumab has approved indications in RCC, NSCLC, MSI-high colorectal cancer and hepatocellular cancer.

However, the toxicity of ipilimumab presents a key challenge to wider adoption.

Prostate cancer and small cell lung cancer are additional potential indications for treatment with a novel anti-CTLA-4 therapy.

Data from Phase 2 trials for ipilimumab showed promising data for treatment with an anti-CTLA-4 agent.

While the Phase 3 trials

for ipilimumab in these indications did not achieve their primary endpoints, data from these trials indicated the potential for treatment with an improved anti-CTLA-4 molecule.

Cytokine Programs

The major focus in our cytokine programs is the development of cytokines with exemplary clinical activity and tolerability.

These programs include XTX202, our optimized IL-2 product candidate, XTX301, our optimized IL-12 product candidate, and XTX401, our optimized IL-15 product candidate.

XTX202, our tumor-selective IL-2 product candidate

XTX202 is a modified form of IL-2 that is masked with a protein domain to prevent binding activity until cleaved off by TME-associated proteases.

In January 2022, we initiated a Phase 1/2 trial to evaluate XTX202 in patients with solid tumors, and enrollment is ongoing.

We expect to present preliminary Phase 1 data in the second half of 2022.

Once we have established an RP2D, we plan to initiate Phase 2 trials evaluating XTX202 as monotherapy in patients with RCC and melanoma, and subject to the results of these trials, we anticipate initiating multiple registration-enabling clinical trials for XTX202 in combination with standard of care agents in a range of indications.

The major focus in our cytokine programs is the development of tumor-activated cytokines with exemplary clinical activity and tolerability.

These cytokine programs include our clinical-stage, tumor-activated product candidates, XTX202 (IL-2) and XTX301 (IL-12), as well as discovery-stage programs for oncology and autoimmune diseases.

XTX202, Our Clinical-Stage, Tumor-Activated IL-2 Product Candidate

XTX202 is an investigational tumor-activated beta-gamma biased (non-alpha), engineered IL-2 molecule designed to potently stimulate CD8+ effector T cells and NK cells without concomitant stimulation of regulatory T cells when activated (unmasked) in the tumor microenvironment.

We are currently evaluating XTX202 as a monotherapy in an ongoing Phase 1 clinical trial in patients with advanced solid tumors.

For more information, please see the sections entitled “—Clinical Development Plans for XTX202” and “—Phase 1 Clinical Trial for XTX202” below.

Background on IL-2

Extensive clinical evidence of the promise and limitations of cytokines

As shown in the figure below, which is a Kaplan-Meier survival analysis, high-dose IL-2 resulted in long-term survival in a subset of patients who had achieved a CR.

As shown in the figure below, high-dose IL-2 has resulted in long-term survival in a subset of patients who had achieved a CR.

We believe that patients who develop a CR when treated with high-dose IL-2 are highly likely to achieve a long-term durable response or cure.

3 unchanged sentences

When administered locally, IL-2 has been shown to be clinically active and well-tolerated, shrinking local cancerous lesions and reducing malignant effusions.

However, when administered systemically, treatment with IL-2 has been shown to induce severe toxicities, including vascular leak syndrome, or VLS, myocardial infarction, or heart attack, acute renal failure and immune-mediated neuropathy.

However, when administered systemically, treatment with IL-2 has been shown to induce severe toxicities, including VLS, myocardial infarction, or heart attack, acute renal failure and immune-mediated neuropathy.

This toxicity profile greatly limits its current use.

3 unchanged sentences

The toxicities associated with early IL-2 therapies, such as aldesleukin, are hypothesized to be associated in part with binding and signaling through the high affinity αβγ IL-2receptor on immune cells or vascular endothelial cells.

In addition, the αβγ IL-2-receptor is expressed at high levels on Tregs, which act to inhibit the immune response, whereas the intermediate affinity βγ IL-2 receptor is expressed on cells that promote immune response including CD8+ T cells and natural killer, or NK, cells.

In addition, the αβγ IL-2receptor is expressed at high levels on regulatory T cells, or TREGs, which act to inhibit the immune response, whereas the intermediate affinity βγ IL-2 receptor is expressed on cells that promote immune response including CD8+ effector T cells and NK cells.

Modeling of IL-2 activity in preclinical animal tumor models and evaluation of dosage and dose-frequency data from patients has suggested that IL-2 has a steep dose-activity curve, with reduced exposure impacting both efficacy and toxicity.

IL-2 anti-tumor activity and toxicity are both dependent on the amount of IL-2 administered.

Therefore, in order to provide the greatest benefit to patients, the goal is to engineer a form of IL-2 that can minimize systemic effects while harnessing and directing activity to the TME.

Therefore, in order

to provide the greatest benefit to patients, the goal is to engineer a form of IL-2 that can minimize systemic effects while harnessing and directing activity to the tumor microenvironment.

Rationale for non-alpha IL-2

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IL-2 is a master regulator of immune responses and has been investigated extensively as a potential anti-cancer immunotherapy.

IL-2 supports the function, survival and proliferation of T cells, including the subset of T cells known as CD8+ T cells that are most closely linked to anti-tumor immunity.

As shown in the figure below, the activities of IL-2 are driven by two classes of receptor complexes, the high-affinity αβγ receptor that contains three subunits and the intermediate-affinity βγ receptor that lacks the α-chain, a receptor known as CD25.

The receptor complexes are differentially expressed on different cell types.

The immune activating CD8+ T cells and NK cells primarily express βγ, and Tregs express the αβγ receptor.

A wild-type IL-2 binds preferentially to the αβγ receptor and therefore strongly stimulates Tregs, thus limiting the immune activating effect of wild-type IL-2.

Moreover, the presence of the high affinity αβγ receptor on Tregs allows these cells to act as scavengers of wild-type IL-2 and reduce the availability of wild-type IL-2 for stimulation of CD8+ or NK cells.

By contrast, non-α IL-2 does not bind to the α receptor component of the αβγ complex and therefore binds equally to CD8+, NK and Treg cells.

This shifts the balance of activity for non-α IL-2 away from Tregs and allows more effective activation of CD8+ and NK cells.

In addition, non-α IL-2 is not subject to scavenging by Tregs since it does not bind to the αβγ high affinity receptor and only binds to the intermediate affinity βγ receptor.

By not being subject to scavenging by Tregs, the levels of non-α IL-2 are not reduced.

Therefore, non-α IL-2 is available at higher concentrations to activate signaling through the βγ receptor on CD8+ and NK cells.

As a result, non-α IL-2 is expected to be more effective at promoting anti-tumor immune response and thus inhibiting tumor growth than a wild-type IL-2.

Avoiding IL-2 Receptor α Reduces Immune Suppression and Supports Immune Activation

Our preclinical data have demonstrated that non-α IL-2 achieved at least the same degree of tumor growth inhibition in mice as a wild-type IL-2, as shown in the figure below.

We compared the ability of non-α IL-2 and wild-type IL-2 to inhibit the growth of established MC38 colon cancer tumors in mice.

Mice received 2 mg/kg of either non-α IL-2 or wild-type IL-2 three times per week and equivalent levels of tumor growth control were achieved.

Notably, this preclinical study compared non-masked versions of wild-type IL-2 and non-α IL-2, which resulted in mortality for a subset of mice after the sixth day in both groups.

In addition to these preclinical data, recent results from a third party’s clinical trial showed objective responses in patients dosed with a systemic non-α IL-2 molecule.

Similar Tumor Growth Inhibition for Wild Type and Non-Alpha IL-2 in Mice with MC38 Colon Cancer

IL-2 supports the function, survival and proliferation of T cells, including the subset of T cells known as CD8+ effector T cells that are most closely linked to anti-tumor immunity.

The activities of IL-2 are driven by two classes of receptor complexes, which are present on different T cell subsets.

The high-affinity αβγ receptor present primarily on TREGs, and the intermediate-affinity βγ receptor present primarily on CD8+ effector T cells and NK cells.

In contrast to wild-type IL-2, XTX202 does not bind the α-subunit of the IL-2 receptor and therefore does not induce the preferential activity on TREGs that limits the immune activating effect of wild type IL-2.

XTX202 is designed to potently stimulate CD8+ effector T cells and NK cells that express the βγ receptor.

Our Solution:

The critical challenge in the development of IL-2 therapeutics is to improve patient tolerability without reducing efficacy.

XTX202, a Tumor-Activated IL-2

The critical challenge in the development of IL-2 therapies is to improve patient tolerability without reducing efficacy.

Deploying the key structural components of our GPS platform, we have designed XTX202 with three key features designed to overcome this:

(1) avoidance of binding to CD25, the IL-2α receptor subunit, in order to reduce the activation of Treg cells that inhibit immune response, while maintaining effective activation of CD8+ and NK cells that promote an anti-tumor immune response;

(2) overcoming the short circulating half-life of the native cytokine using the half-life extension

and (3) a removable protease-cleavable protein mask that prevents XTX202 from binding and signaling until the mask is removed by the MMPs that are preferentially active within the TME.

These key features are intended to ensure that XTX202 is released and activated preferentially within the TME, where it has been designed to bind to lymphocytes.

In the TME, XTX202 is designed to be unmasked and to bind to the IL-2 βγ receptors that are abundantly expressed on CD8+ T effector cells and NK cells, activating these cells.

(2) overcoming the short circulating half-life of the native cytokine using the half-life extension domain;

and (3) a removable protease-cleavable protein mask that prevents XTX202 from binding and signaling until the mask is removed by the MMPs that are preferentially active within the tumor microenvironment.

These key features are intended to ensure that XTX202 is released and activated preferentially within the tumor microenvironment, where it has been designed to bind to lymphocytes.

In the tumor microenvironment, XTX202 is designed to be unmasked and to bind to the IL-2 βγ receptors that are abundantly expressed on CD8+ effector T cells and NK cells, activating these cells.

Locally activated T cells and NK cells have potent anti-tumor cytotoxic activity.

The unmasked XTX202 is then rapidly internalized by these lymphocytes, shortening the systemic half-life of the unmasked molecule.

The unmasked XTX202 is then rapidly internalized by these lymphocytes, shortening the systemic half-life of the active (unmasked) molecule and localizing its activity to the tumor.

Overview of preclinical studies and data

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This was done to assess the activity of our masking technology and compare XTX202 against high-dose IL-2.

We believe that these studies collectively provide preclinical POC with an improved therapeutic index due to TME-dependent activation of XTX202 that, if replicated in clinical trials, could result in significant benefits to patients with a variety of cancer types.

We believe that these studies collectively provide preclinical proof-of-concept with an improved therapeutic index due to tumor microenvironment-dependent activation of XTX202 that, if replicated in clinical trials, could result in significant benefits to patients with a variety of cancer types.

The key findings of our XTX202 preclinical studies are as follows:

● XTX202 did not signal through the IL-2 receptor when masked, and MMP activation of XTX202 restored full potency of IL-2 signaling in in vitro assays illustrating the tight, protease-dependent control of IL-2 activity conferred by XTX202;

XTX202 demonstrated minimal signaling through the IL-2 receptor when masked, and MMP activation of XTX202 restored full potency of IL-2 signaling in in vitro assays illustrating the tight, protease-dependent control of IL-2 activity conferred by XTX202;

XTX202 activated an immune response in the tumor but not in peripheral organs, demonstrating geographically selective tumor PD activity in vivo in mice;

● XTX202 matched the tumor growth inhibition activity of XTX200 and aldesleukin, without activation of immune response outside of the TME, thereby avoiding the systemic toxicity (VLS and enlargement of the spleen, or splenomegaly), body weight loss and mortality in mice that were associated with the doses of XTX200 or aldesleukin required for tumor growth inhibition;

● XTX202 was well-tolerated in NHPs with no evidence of systemic immune activation outside of the tumor or VLS, whereas XTX200 induced both;

XTX202 matched the tumor growth inhibition activity of XTX200 and aldesleukin, without activation of immune response outside of the tumor microenvironment, thereby avoiding the systemic toxicity (VLS and enlargement of the spleen, or splenomegaly), body weight loss and mortality in mice that were associated with the doses of XTX200 or aldesleukin required for tumor growth inhibition;

XTX202 was well-tolerated in non-human primates, or NHPs, with no evidence of VLS or systemic immune activation in non-tumor, healthy tissue, whereas XTX200 induced both;

XTX202 exhibited a half-life of greater than one week in NHPs, compared to a half-life of less than 0.5 days for XTX200.

Based on these data, we believe that circulating levels of masked XTX202 can be achieved with clinically meaningful concentrations of activated (unmasked) XTX202 within tumors and that any unmasked cytokine of XTX202 that reaches the systemic circulation will be rapidly cleared with minimal systemic AEs.

We evaluated the masking feature in XTX202 using a sensitive, in vitro assay that measures IL-2 activity.

In our study, we monitored the IL-2 dependent activation of the JAK-STAT pathway, a chain of interactions between proteins in a cell involved in immunity, using HEK-Blue IL-2 reporter cells that expressed IL-2Rαβγ, JAK and STAT genes.

Avoiding activation, or phosphorylation, of STAT5, called pSTAT5, would suggest potential for reduced systemic toxicity.

In our study, pSTAT5 activity is expressed as absorbance at 625 nanometers.

As shown in the figure below, reporter cells were incubated with recombinant human IL-2, or rhIL-2, XTX202, or XTX202 that was incubated with an MMP.

Both the positive control rhIL-2 and MMP-treated XTX202 activated the reporter cell line at a dose of 50 pM and induced pSTAT5 activity.

Conversely, XTX202 induced minimal pSTAT5 activity at 57 pM.

Based on these data, we concluded that masked XTX202 did not signal through IL-2 receptor and that MMP activation of XTX202 restored full potency of IL-2 signaling.

XTX202 Provided Tight, Protease-Dependent Control of IL-2 Activity

We then used syngeneic mouse tumor models to evaluate the activity of XTX202 in vivo .

In these mouse tumor models, we observed tumor-selective PD effects of XTX202 and robust monotherapy activity against established tumors.

Importantly, these experiments were conducted with the XTX202 and not mouse surrogates.

In these mouse tumor models, we observed tumor-activated PD effects of XTX202 and robust monotherapy activity against established tumors.

Importantly, these experiments were conducted with XTX202 and not mouse surrogates.

We examined established B16F10 melanoma tumors for PD evidence of XTX202 activity and compared the results to activity seen systemically.

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Animals received 10 mg/kg of XTX202 or 0.36 mg/kg of XTX200, the non-masked control, on day zero and day three.

The presence of CD8+ T cells was used as a measure of immune activation.

The presence of CD8+ effector T cells was used as a measure of immune activation.

The figure below depicts activity in B16F10 melanoma tumors on the left and in the spleen, a peripheral immune organ, on the right.

We observed that both XTX202 and XTX200 induced an increase in CD8+ T cells in the tumor compared to vehicle control, while, in the spleen, expansion observed following XTX202 was limited compared to non-masked XTX200, which induced a significant expansion of splenic CD8+ T cells.

We observed that both XTX202 and XTX200 induced an increase in CD8+ effector T cells in the tumor compared to vehicle control, while, in the spleen, expansion observed following XTX202 was limited compared to non-masked XTX200, which induced a significant expansion of splenic CD8+ effector T cells.

XTX202 Tumor Selectivity in B16F10 Tumor Mice

In a separate experiment, we compared the effect of aldesleukin and XTX202 on tumor growth and the level of intra-tumoral CD8+ effector T cell, NK cell and TREG infiltration.

MC38 tumor-bearing mice were treated with either vehicle, aldesleukin (3 mg/kg twice per day, or BID) or XTX202 (10 mg/kg once every five days, or Q5D) and tumor volumes, as well as intra-tumoral immune cell infiltration were assessed on day five post-treatment.

As illustrated in the figure below, XTX202 and aldesleukin treatment resulted in tumor growth inhibition, as well as increased CD8+ effector T cell and NK cell infiltration compared to vehicle.

In contrast to aldesleukin, XTX202 treatment did not result in significantly increased intra-tumoral TREG frequencies compared to vehicle.

In a separate experiment, we compared the ability of XTX202, XTX200 and aldesleukin to inhibit growth of established MC38 colon cancer tumors in mice.

Prior experiments had demonstrated that 0.5 mg/kg of XTX200 dosed once every three days, or Q3D, and 3 mg/kg of aldesleukin dosed twice per day, or BID, for five days were the maximum tolerated doses, or MTDs, in mice for these molecules.

Prior experiments had demonstrated that 0.5 mg/kg of XTX200 dosed once every three days, or Q3D, and 3 mg/kg of aldesleukin BID for five days were the maximum tolerated doses, or MTDs, in mice

for these molecules.

Previous experiments had also demonstrated that XTX202 was well-tolerated in mice at doses up to at least 25 mg/kg dosed Q3D, with reversible, mild decrease in body weight and no mortality, and no decrease in body weight at 10 mg/kg.

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These samples were evaluated for changes in immune cell populations in the blood by fluorescent activated cell sorting.

XTX202 did not cause an increase in circulating CD8+ T cells, NK cells or Tregs.

XTX202 did not cause an increase in circulating CD8+ effector T cells, NK cells or TREGs.

In contrast, XTX200 caused an increase in circulating CD8+ and NK cells, but no effect on TREGs, and aldesleukin had variable but stimulatory effects on all three cell types.

XTX202 Did Not Affect Immune Cells in the Periphery

These preclinical data collectively support the hypothesis that the activity of XTX202 is limited to the TME.

The lack of expansion of the CD8+ T cell, NK cell or Treg cell populations in the peripheral blood suggest that XTX202 achieved tumor growth inhibition while exhibiting no evidence of activity outside of the TME.

By contrast, the non-masked molecule XTX200 and aldesleukin both showed significant increases in immune cell populations in the blood, demonstrating that these molecules are both active outside of the TME.

These preclinical data collectively support the hypothesis that the activity of XTX202 is limited to the tumor microenvironment.

This conclusion was further supported by measurements of animal health in the bladder cancer tumor model.

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XTX202 Overcame Toxicity of Non-Masked XTX200 in NHPs

XTX202 was evaluated in a repeat-dose GLP toxicology study in NHPs.

XTX202 was evaluated in a repeat-dose toxicology study in NHPs in compliance with FDA’s good laboratory practices, or GLPs.

The study evaluated vehicle and 1, 10 and 25 mg/kg doses of XTX202 given once weekly intravenously.

As shown in the figure below, we observed dose dependent lymphocyte expansion following administration of XTX202, with the 1 mg/kg group showing no systemic expansion.

These results were further supported by analysis of circulating albumin a decrease in serum albumin levels is an indication of VLS.

These results were further supported by analysis of circulating albumin.

A decrease in serum albumin levels is an indication of VLS.

Serum albumin remained within the normal range for the 1 mg/kg and 10 mg/kg dosing groups, indicating no systematic expansion occurred and demonstrating minimal or no systemic toxicity for XTX202 in those dosing groups.

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As shown in the table below, XTX202 exhibited a half-life of 5.3 days, whereas the half-life of the non-masked XTX200 was shorter at 1 day.

The half-life extension moiety and the decrease in target-mediated

disposition due to masking of IL-2 result in the longer half-life of XTX202.

These data support starting with a dosing schedule of intravenous administration once every three weeks.

The half-life extension moiety and the decrease in target-mediated disposition due to masking of IL-2 result in the longer half-life of XTX202.

These data support starting with a dosing schedule of intravenous administration once every three weeks, or Q3W.

Measured PK Parameters in Single Dose NHP Study

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Based on these data, we believe that circulating levels of masked XTX202 can be achieved with clinically meaningful concentrations of activated (unmasked) XTX202 within tumors and that any unmasked cytokine of XTX202 that reaches systemic circulation will be rapidly cleared with no systemic AEs.

We believe that our preclinical data support clinical development of XTX202 for the treatment of a range of cancer indications.

We believe that our preclinical data support clinical development of XTX202 for the treatment of a range of cancer indications, both as a monotherapy and in combination with other agents.

In head-to-head preclinical tumor model studies, XTX202 achieved activity with 2-10 mg/kg dosed Q3D comparable to aldesleukin at 3 mg/kg dosed BID.

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We expect this to allow dose-escalation to achieve intra-tumor cytokine levels high enough to induce local T cell and NK cell activation, proliferation and anti-tumor cytotoxicity.

Clinical Development Plan

In January 2022, we initiated a Phase 1/2 trial to evaluate XTX202 in patients with solid tumors, and enrollment is ongoing.

We expect to present preliminary Phase 1 data in the second half of 2022.

We aim to establish safety POC by showing XTX202 has the ability to achieve target exposure at or below the RP2D without leading to significant AEs in patients.

An additional objective is to show efficacy POC with anti-tumor activity at the RP2D by evaluating intra-tumor PD activity for evidence of IL-2 activity in the tumor.

Assuming we successfully complete the Phase 1 trial and determine the RP2D for XTX202, we plan to initiate Phase 2 expansion cohorts with XTX202 monotherapy to determine the objective response rate, including CRs, in RCC and melanoma patients who previously received an anti-PD-1 treatment regimen.

Subject to the response rate observed in the POC phase and discussions with regulatory authorities, we plan to initiate non-randomized monotherapy trials with the potential for accelerated approval.

In addition, we plan to initiate one or more additional Phase 1 trials aimed at demonstrating that we can combine XTX202 with other agents such as anti-PD-1 agents or tyrosine kinase inhibitors, or TKIs, which are small molecules that are designed to inhibit the activation of enzymes called tyrosine kinases that are drivers of certain cancers and proliferative disorders.

If we are able to generate objective responses in patients with RCC or melanoma, we plan to initiate registration-enabling clinical trials in the indication or indications relevant to these patients.

Beyond RCC and melanoma, we also plan to explore the potential of XTX202 in other solid tumor indications.

In addition to the monotherapy potential for XTX202, we believe the combination of XTX202 with additional agents, including anti-PD-1/PD-L1 agents that are widely used in a range of solid tumors, has the potential to ultimately lead to the widest utilization of XTX202 in cancer patients.

Based on preclinical data, we believe XTX202 has the potential to provide enhanced anti-tumor activity when combined with an anti-PD-1/PD-L1.

As shown in the figure below, in preclinical studies, XTX202 combined with an anti-PD-1/PD-L1 induced complete responses in a preclinical setting where neither XTX202 nor the anti-PD-1/PD-L1 were able to achieve complete responses as a monotherapy.

In these preclinical studies, mice with complete responses rejected rechallenge with tumor, indicating evidence of the development of a memory response.

Anti-tumor activity of XTX202 as a single agent and in combination with anti-mPD-1 was evaluated in hFcRn Tg32 transgenic mice bearing the murine MB49 bladder carcinoma model.

The combination of XTX202 with anti-mPD-1 further improved anti-tumor activity with TGI 92% on Day 13 (Data presented as mean ±SEM, two-way ANOVA followed by post hoc Dunnett’s test, *P < 0.05;

**P < 0.005).

The treatment with XTX202 alone or in combination with anti-mPD-1 improved animal survival from 19 days to 27.5 and 38 days, respectively (Geham-Breslow-Wilcoxon test, **P < 0.01).

A mouse with complete regression of MB49 tumor after combination therapy with XTX202 and anti-mPD-1 was resistant to tumor rechallenge with autologous MB49 tumor implanted on the opposite flank.

Clinical Development Plans for XTX202

We are currently evaluating XTX202 in monotherapy dose-escalation (Part 1A) in an ongoing Phase 1 clinical trial in patients with advanced solid tumors, and monotherapy dose expansion (Part 1B) is open for enrollment.

The Phase 1 trial is a first-in-human, multi-center, open-label trial designed to evaluate the safety and tolerability of multiple ascending doses of XTX202 with the goal of establishing recommended Phase 2 dose, or RP2D, for XTX202 as a monotherapy in patients with advanced solid tumors.

We plan to initiate patient enrollment in a Phase 2 clinical trial evaluating XTX202 as a monotherapy in patients with unresectable or metastatic melanoma and metastatic RCC in April 2023.

The planned Phase 2 clinical trial for XTX202 is a multi-center, open-label trial designed to evaluate the safety and anti-tumor activity of XTX202 as a monotherapy in patients with unresectable or metastatic melanoma and metastatic RCC at the RP2D.

Subject to the results from this combination study, we anticipate initiating registration-enabling clinical trials for XTX202 in combination with an anti-PD-1/PD-L1 in patients with NSCLC.

The figure below shows our Phase 1/2 trial design for XTX202 in RCC, melanoma and other indications to evaluate its activity as both a monotherapy and in combination with other agents.

Phase 1/2 Trial Designed to Establish POC with Potential to Trigger Accelerated Approval Cohort for Monotherapy

Phase 1 Clinical Trial for XTX202

As of March 1, 2023, a total of 16 patients have been treated with XTX202 in the outpatient setting at five dose levels ranging from 0.27 mg/kg to 1.4 mg/kg Q3W in Part 1A for the Phase 1 clinical trial, and Part 1B is open for enrollment.

No signs of VLS, including hypotension or decreases in albumin (an early sign of VLS) or hemodynamic compromise, have been

observed in patients to date.

A maximum tolerated dose has not yet been determined, and enrollment in Part 1A and Part 1B of the clinical trial is ongoing.

In addition, we recently reported preliminary clinical intra-tumoral PD data for two patients treated with XTX202 who each had an optional on-treatment tumor biopsy and were the only two patients for whom a tumor biopsy analysis was available as of March 1, 2023.

The first patient was a melanoma patient treated with XTX202 at the 0.38 mg/kg dose level, and the second patient was an RCC patient treated with XTX202 at the 0.53 mg/kg dose level.

For each patient, as shown in the figures below, the tumor biopsy featured increased numbers of stromal TILs and increased frequency of CD8+ effector T cells among these TILs.

Importantly, in each patient, at the time of the tumor biopsy, these changes occurred in the absence of peripheral changes to either CD8+ effector T cells, demonstrating preliminary evidence of tumor-selective activation.

Renal Cell Carcinoma (RCC) Overview

RCC accounts for 2% of global cancer diagnoses and deaths annually, and the global incidence is increasing.

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Survival is highly dependent on the stage at diagnosis, with metastatic disease having a five-year survival rate of only 15%.

RCC is the ninth most common cancer in the United States, with approximately 74,000 new cases and approximately 15,000 deaths projected in 2020.

RCC is the ninth most common cancer in the United States, with approximately 79,000 estimated new cases in 2022 and approximately 13,920 deaths in 2022.

The landscape of therapeutic options has rapidly evolved such that the treatment goal, even in the metastatic setting, is to cure patients or ensure their long-term survival.

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but Most Patients Still Relapsed Within 18 Months

In contrast to the TKI and anti-PD-1 combination shown above, IL-2-directed therapy offers the opportunity for durable responses and cures, as was seen in the historical aldesleukin treatment data.

In contrast to the TKI and anti-PD-1/PD-L1 combination shown above, IL-2-directed therapy offers the opportunity for durable responses and cures, as was seen in the historical aldesleukin treatment data.

Only 15% of RCC patients treated with high-dose IL-2 obtained an objective response but approximately half of these, or 7% of all patients treated, achieved a CR.

The unique feature of high-dose IL-2 is that approximately 90% of patients with RCC who achieved a CR remained permanently disease-free and off-treatment.

Assuming a successful Phase 1 dose escalation trial and identification of a RP2D, we plan to pursue a rapid Phase 2 POC clinical trial in RCC, an indication in which recombinant IL-2 produces positive clinical responses, but in which its use is limited due to toxicity.

Assuming a successful Phase 1 dose escalation trial and identification of a RP2D, we plan to pursue a rapid Phase 2 proof-of-concept clinical trial in RCC, an indication in which recombinant IL-2 produces positive clinical responses, but in which its use is limited due to toxicity.

We believe XTX202’s characteristics can overcome these limitations and address the unmet need in RCC.

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In clinical trials evaluating recombinant IL-2 for the treatment of metastatic melanoma, recombinant IL-2 produced an ORR of 16% and produced CRs and functional cures in 6% of patients.

However, use of recombinant IL-2 has been limited by dose-limiting toxicities.

However, use of recombinant IL-2 has been hampered by dose-limiting toxicities.

While treatments that target PD-1 and CTLA-4 have grown to dominate the metastatic melanoma treatment landscape, many patients do not respond, and relapses are common, with a five-year survival rate of around 50% in metastatic melanoma.

We believe that a safe and effective form of IL-2 may improve initial response rates and clinical outcomes when added to checkpoint inhibitor therapy and may maintain responses in patients with melanoma who have relapsed from checkpoint inhibitor treatment.

Importantly, high-dose IL-2 has shown a response rate, including CRs in patients with melanoma despite those patients having progressed on prior treatment with an anti-PD-1 showing the potential for IL-2 mechanism of action-based efficacy in patients who have previously been treated with an anti-PD-1.

Similar to our development plan for RCC, assuming a successful Phase 1 dose escalation trial, we plan to rapidly pursue a Phase 2 POC clinical trial in melanoma, where recombinant IL-2 has demonstrated clinical responses but has been historically limited due to toxicity.

Our clinical development plan includes the evaluation of patients with melanoma who have previously been treated with an anti-PD-1/PD-L1 to demonstrate clinical POC.

Importantly, high-dose IL-2 has shown a response rate, including CRs in patients with melanoma despite those patients having progressed on prior treatment with an anti-PD-1/PD-L1 showing the potential for IL-2 mechanism of action-based efficacy in patients who have previously been treated with an anti-PD-1/PD-L1.

Similar to our development plan for RCC, assuming a successful Phase 1 dose escalation trial, we plan to rapidly pursue a Phase 2 proof-of-concept clinical trial in melanoma, where recombinant IL-2 has demonstrated clinical responses but has been historically limited due to toxicity.

Our clinical development plan includes the evaluation of patients with melanoma who have previously been treated with an anti-PD-1/PD-L1 to demonstrate clinical proof-of-concept.

Potential Future Indications

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Dose-limiting toxicities have prevented most patients with cancer from receiving the high doses necessary for systemic efficacy.

Multiple clinical trials have evaluated IL-2 where local treatment is possible.

Multiple clinical trials have evaluated IL-2 where localized administration is possible.

In these trials, IL-2 has induced objective responses in patients with ovarian cancer when administered in the peritoneum, in patients with bladder cancer when administered directly into the bladder and in patients with NSCLC and mesothelioma when administered into the pleural cavity.

In patients with resectable HNSCC, perioperative administration of IL-2 into the lymph nodes near the tumor resulted in increased survival in a randomized trial.

These data provide POC that multiple tumor types are likely sensitive to IL-2 if high levels of exposure can be obtained.

XTX301, our half-life extended, tumor-selective IL-12 therapeutic

XTX301 is an engineered form of IL-12 that is masked with a protein domain to prevent binding activity until cleaved off by TME-associated proteases.

In preclinical studies, we observed that XTX301 was activated in a protease-dependent manner, including in preclinical studies using human tumor samples ex vivo showing XTX301 was able to cleave the masking domain.

XTX301 is currently in IND-enabling studies, and we plan to submit an IND to the FDA in the second half of 2022 for evaluation in patients with solid tumors.

These data provide evidence that multiple tumor types are likely sensitive to IL-2 if high levels of localized exposure can be obtained.

In addition, we believe the novel design of XTX202 and preclinical and clinical data observed to date support the potential for XTX202 as a treatment in combination with other agents where tumor-activation may be required in order to combine with IL-2, including:

monoclonal antibodies, other pro-inflammatory cytokines such as IL-12 and IL-18, cell therapies such as CAR-T and CAR-NK, and cancer vaccines.

XTX301, Our Clinical-Stage, Tumor-Activated IL-12 Product Candidate

In November 2022, we announced clearance of our IND by the FDA for XTX301 for evaluation in patients with advanced solid tumors.

For more information, please see the section entitled “—Clinical Development Plans for XTX301” below.

Background on IL-12

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IL-12 is a key cytokine in the body’s response to pathogen infection, sending a signal to T cells, among others.

IL-12 interacts with diverse immune cells, including CD4+ T cells, CD8+ T cells, NK cells, monocytes and macrophages.

IL-12 interacts with diverse immune cells, including CD4+ T cells, CD8+ effector T cells, NK cells, monocytes and macrophages.

IL-12’s broad range of pro-inflammatory functions suggests that it could potentially be highly potent in controlling anti-cancer immunity.

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Therefore, to unleash the potential for IL-12 in the majority of patients with advanced or metastatic cancer, an IL-12 that can be administered systemically but act locally at the tumor site is needed.

The failure of systemic IL-12 to induce meaningful anti-tumor efficacy is generally attributed to tolerability, which limits the dose and, as a result, the ability to reach therapeutic concentrations within the TME.

Therefore, maximizing the amount of IL-12 that reaches the tumor, while minimizing exposure of non-tumor tissue, may be critical for a safe and effective anti-tumor response.

Tumor-selective activity is therefore a desirable therapeutic profile.

Tumor-selective activation is therefore a desirable therapeutic profile.

Our Solution:

Our goal for our IL-12 program is to create a tumor-selective, extended half-life IL-12 therapeutic with minimal peripheral effects.

We are using our GPS platform and proprietary approach to achieve systemic delivery of tumor-selective IL-12, which we believe would have potential as a monotherapy and in combination with other therapies.

XTX301, a Tumor-Activated IL-12

Our goal for our IL-12 program is to create a tumor-activated, extended half-life IL-12 therapeutic with minimal peripheral effects.

We are using our GPS platform and proprietary approach to achieve systemic delivery of tumor-activated IL-12, which we believe would have potential as a monotherapy and in combination with other therapies.

The design of our masked IL-12 cytokine molecule is closely related to that of our masked IL-2 cytokine molecule, which illustrates the flexibility and robustness of our cytokine engineering approach.

The masking domain is designed to prevent binding to the cell-surface IL-12 receptor, unless the linker containing the protease site is cleaved by proteases preferentially active in the TME.

The masking domain is designed to prevent binding to the cell-surface IL-12 receptor, unless the linker containing the protease site is cleaved by proteases preferentially active in the tumor microenvironment.

The half-life extension domain is designed to overcome the short circulating half-life of the native cytokine and the overall molecule is designed to enhance the efficiency of manufacturing.

Overview of preclinical studies and data

We have observed tumor-selective activity of XTX301 in mouse models after systemic administration.

Anti-tumor activity was observed in syngeneic mouse models using murinized surrogate molecules.

In order to achieve anti-tumor activity in vivo , we designed XTX301 to optimize its circulating half-life while maintaining masking so that little or no activity was observed in non-tumor tissue.

We observed minimal effects outside the tumor in mouse models.

In a preliminary repeat-dose NHP study, XTX301 was observed to be well-tolerated at doses at least 50-fold higher than a non-masked human IL-12 control.

As shown in the figure below, XTX301 did not signal through IL-12 receptors when masked, and MMP activation of XTX301 restored full potency of IL-12 signaling.

Reporter cells that express the IL-12 receptors were incubated with recombinant IL-12, XTX301 alone or XTX301 that was incubated with an MMP.

We observed that both the positive

control rhIL-2 and MMP-treated XTX301 activated the reporter cell line and induced phosphorylation of STAT4.

XTX301 induced minimal phospho-STAT4 activity.

The effective masking and protease-dependent activity of XTX301 was further confirmed in human primary peripheral mononuclear cells.

XTX301 Provided Tight, Protease-Dependent Control of IL-12 Activity

In preclinical models with mXTX301, a mouse surrogate for XTX301, tumor growth inhibition was observed in melanoma and bladder cancer models without adverse events.

Mouse studies were conducted with a surrogate for XTX301 that included mouse sequences for IL-12 and the masking domain in place of the human sequences present in XTX301 but had the same half-life extension domain and protease linker.

A mouse surrogate was used as human IL-12 does not activate mouse IL-12 receptors.

Mice bearing established tumors received a single dose of 0.38 mg/kg of the non-masked mouse IL-12 cytokine or 0.5 mg/kg or 3.0 mg/kg of mXTX301, and tumor growth was measured over time.

As shown in the figure below, we observed that mXTX301 controlled MC38 tumor growth at a tolerable single dose of 0.5 mg/kg or 3 mg/kg compared to the vehicle control.

mXTX301 achieved tumor regressions in a subset of the animals dosed.

Similarly, we observed that the non-masked IL-12 control resulted in tumor growth inhibition.

However, a tolerated dose of the non-masked control was not established in this study, and the dose required for tumor growth inhibition resulted in animal mortality by day six.

These data suggest that mXTX301 is highly active and can achieve tumor regressions in mice at a tolerable dose.

In vivo activity was further confirmed in additional syngeneic mouse tumor models, including MB49 and B16F10.

Mouse Surrogate Masked IL-12, mXTX301, is Active and Well-Tolerated In Vivo

We also evaluated pharmacodynamic effects of mXTX301 treatment in syngeneic mouse tumor models.

Animals with established MC38 or B16F10 tumors were treated with either vehicle, 0.5 mg/kg mXTX301 or 0.38 mg/kg non-masked control (mXTX300).

Treatment with mXTX301 resulted in intra-tumoral induction of IFNg in B16F10 and MC38 tumor bearing mice with low peripheral exposure.

In contrast, treatment with the non-masked control molecule, mXTX300, resulted in significant amounts of peripheral interferon gamma detected in plasma.

Mouse Surrogate Masked IL-12, mXTX301, Elicits Pharmacodynamic Effects in Tumors

and Exhibits Effective Peripheral Masking in Mouse Models

In NHPs, XTX301 and XTX300 (unmasked) were dosed once weekly intravenously for four doses.

XTX301 was dosed at 3 mg/kg for the first dose and then dropped to 1.5 mg/kg for the subsequent three doses.

XTX301 showed minimal systemic activity and was observed to be well-tolerated at doses up to 1.5 mg/kg.

The XTX301 dose was at least 50-fold higher than the MTD of XTX300, which was less than 0.03 mg/kg in this preclinical study.

As shown in the figures below, in NHPs, the non-masked control demonstrated peripheral activation of CD8+ T cells and NK cells, whereas XTX301 was similar to vehicle with minimal activation of CD8+ T cells and NK cells.

Current Status and Clinical Development Plan

Based on these data, we are currently advancing XTX301 through IND-enabling studies including cell line development and manufacturing, and a non-clinical toxicology program to support our planned IND submission and progression into clinical development.

We plan to submit an IND to the FDA in the second half of 2022 for evaluation in patients with solid tumors.

We have undertaken extensive preclinical studies that demonstrate the ability of XTX301 to induce potent anti-tumor activity and PD changes consistent with known IL-12 biology.

Importantly, these effects on the tumor were observed without inducing concomitant systemic toxicity, which provides evidence that the masking design is performing as intended in preclinical studies.

In murine preclinical models, we used a murine surrogate for XTX301, which we refer to as mXTX301, which included mouse sequences for IL-12 and the masking domain in place of the human sequences present in XTX301, since human IL-12 does not activate mouse IL-12 receptors.

mXTX301 is architecturally analogous to XTX301 and shares the same half-life extension domain and protease linker as XTX301.

Collectively, our preclinical data showed that:

XTX301 demonstrated minimal signaling through the IL-12 receptor when masked, and MMP activation of XTX301 restored potent IL-12 signaling in in vitro assays, illustrating the tight, protease-dependent control of IL-12 activity enabled by XTX301;

mXTX301 treatment induced potent anti-tumor activity in response to single dose administration in multiple tumor models without inducing significant body weight loss and toxicity;

mXTX301 treatment resulted in increased infiltration of CD8+ effector T cells into tumors and induced broad pro-inflammatory gene expression changes in tumors consistent with IL-12 biology and its ability to reprogram the tumor microenvironment towards a more immune-permissive state;

XTX301 was tolerated in NHPs at doses up to 2 mg/kg once weekly for four doses, or QWx4, and demonstrated a circulating half-life of four days.

In preclinical models with mXTX301, tumor growth inhibition was observed in melanoma and bladder cancer models without AEs.

Mice bearing established tumors received a single dose of mXTX301 at doses of 0.039 mg/kg, 0.13 mg/kg or 0.39 mg/kg or received 0.038 mg/kg of the non-masked mouse IL-12 cytokine, and tumor growth was measured over time.

As shown in the figure below, mXTX301 demonstrated anti-tumor activity in all doses tested, including complete regressions at the 0.13 mg/kg and 0.39 mg/kg dose levels in a subset of the mice dosed.

While the non-masked mouse IL-12 cytokine also induced complete regressions, severe weight loss and mortality were observed, as compared to a lack of toxicity observed in all of the mice treated with mXTX301 across dose levels.

0.5x106 cells;

single IV dose of mXTX301 and mXTX302 on Day 0.

Tumor growth data shown as mean±SEM.

Tumor volume data was assessed by a two-way ANOVA followed by Bonferroni post hoc test on Day 11 compared to vehicle treated animals.

****p<0.0001 for all mXTX301 treatment groups.

Body weight data are shown as mean ±SEM.

A two-way ANOVA followed by Bonferroni post hoc test compared to vehicle treated animals was performed **p<0.005, ****p<0.0001.

Additional experiments confirmed the anti-tumor activity of mXTX301 across a range of mouse models including MC38, CT26 and B16F10.

As shown in the figure below, a single dose of mXTX301 treatment resulted in marked anti-tumor activity in all models, including complete regressions in a subset of animals with MC38 or CT26 tumors and transient regressions in a subset of mice with B16F10 tumors, which feature a “cold” non-inflamed tumor microenvironment.

To gain deeper insight into the mechanism-of-action of XTX301, we conducted a series of experiments focused on measuring the effect of mXTX301 treatment on the tumor microenvironment in the MC38 model.

The results of these experiments are summarized in the figure below.

Consistent with known IL-12 biology, mXTX301 treatment resulted in increased infiltration of CD8+ effector T cells into tumors.

Additionally, mXTX301 induced robust gene expression changes indicative of broad immune activation in tumors including changes tied to infiltration of diverse immune cell subtypes, interferon gamma signaling and enhancement of antigen presentation.

In addition, preclinical data for XTX301 support the potential to achieve a broad therapeutic index.

As shown in the figure below, in preclinical studies, mXTX301 induced tumor regressions in a murine model following a single dose of mXTX301 at 0.13 mg/kg, and XTX301 was tolerated at doses up to 2.0 mg/kg dosed once every week, or Q1W, four times in NHPs.

Measured PK Parameters in Single Dose NHP Study

XTX301 (2.0 mg/kg)

Clinical Development Plans for XTX301

In November 2022, we announced clearance of our IND by the FDA for XTX301 for evaluation in patients with advanced solid tumors.

We anticipate reporting preliminary safety data from the Phase 1 clinical trial into at least the third dose level in the fourth quarter of 2023.

Our clinical development plan for XTX301 will focus initially on monotherapy dose-escalation in patients with solid tumors.

Following an assessment of anti-tumor effect as a monotherapy in selected tumor types, we also plan to explore the potential to combine XTX301 with other agents prior to initiation of registration-enabling clinical trials.

XTX401, our half-life extended, tumor-selective IL-15 therapeutic

XTX401 is a modified form of IL-15 that is masked with a protein domain designed to prevent binding activity until cleaved off by TME-associated proteases.

In preclinical studies, we observed that XTX401 was activated in a protease-dependent manner, including in preclinical studies using human tumor samples ex vivo showing XTX401 was able to cleave the masking domain, clinically active and well-tolerated.

XTX401 is currently in preclinical development undergoing IND-enabling studies.

Background on IL-15

IL-15 is a cytokine that primarily stimulates the proliferation and cytotoxic functions of CD8+ T cells and NK cells that can enhance anti-tumor responses.

IL-15 activities have some similarities to IL-2, and the receptors bound by the two cytokines partly overlap:

IL-15 signals through a receptor complex composed of the IL-2/IL-15 receptor β chain, or CD122, and the common gamma chain γC, or CD132.

IL-15 signaling induces the proliferation, survival and differentiation of T cells and NK cells, including CD8+ memory T cells that are believed to be critical for durable anti-tumor immunity.

The challenge associated with IL-15 as a potential therapeutic is that it has a short in vivo half-life and has shown considerable toxicity both in preclinical models and in patients.

In NHPs, IL-15 has shown severe toxicity at higher doses, including weight loss and skin rash.

In patients with cancer, severe toxicity was observed when NCI-rhIL-15 was dosed by continuous IV infusion, including a drug-related death.

AEs included hypotension, thrombocytopenia, liver injury and high fever.

More recent versions of IL-15 have caused grade 3 AEs, including lymphocytopenia, increased liver enzymes, anemia, diarrhea and peripheral edema, with minimal clinical activity reported.

Much of the reported toxicity is associated with systemic activation and proliferation of NK cells;

therefore, local administration has been tried with a half-life extended recombinant IL-15, which demonstrated objective responses in patients with bladder cancer in combination with the standard-of-care agents.

The ability to induce local tumor regressions support that IL-15 activity in the TME can have therapeutic benefit provided that systemic toxicity can be avoided.

Subject to the successful completion of monotherapy dose-escalation, we anticipate evaluating initial PD and anti-tumor activity data in cohorts of patients focused on tumor types that are sensitive to treatment with I-O agents and a separate cohort of patients with tumor types historically insensitive to treatment with I-O agents.

The figure below shows our planned Phase 1/2 trial design for XTX301 to enable multiple monotherapy and combination opportunities for expansion in both hot and cold solid tumors.

XTX101, Our Clinical-Stage, Fc-Enhanced Tumor-Activated Anti-CTLA-4 Product Candidate

We are currently evaluating XTX101 in an ongoing Phase 1 clinical trial in patients with advanced solid tumors.

For more information, please see the section entitled “—Clinical Development Plans for XTX101” below.

As previously announced, we plan to continue to explore strategic opportunities to advance XTX101 with a partner beyond the current Phase 1 clinical trial.

Background on CTLA-4

CTLA-4 is an immune checkpoint protein that is well-established as playing a central role in the development of tumors.

The scientific insight that led to the early development of CTLA-4 therapeutics is attributable to investigators recognizing CTLA-4 as a protein on T cells that acts as a brake on T cell activation.

By removing this brake, T cells were freed to

attack cancer.